diff --git a/includes/file.inc b/includes/file.inc
index de05c799fa9..59ecadfe3ec 100644
--- a/includes/file.inc
+++ b/includes/file.inc
@@ -1098,6 +1098,12 @@ function file_space_used($uid = NULL, $status = FILE_STATUS_PERMANENT) {
  * @param $validators
  *   An optional, associative array of callback functions used to validate the
  *   file. See file_validate() for a full discussion of the array format.
+ *   If no extension validator is provided it will default to a limited safe
+ *   list of extensions which is as follows: "jpg jpeg gif png txt
+ *   doc xls pdf ppt pps odt ods odp". To allow all extensions you must
+ *   explicitly set the 'file_validate_extensions' validator to an empty array
+ *   (Beware: this is not safe and should only be allowed for trusted users, if
+ *   at all).
  * @param $destination
  *   A string containing the URI $source should be copied to.
  *   This must be a stream wrapper URI. If this value is omitted, Drupal's
@@ -1160,28 +1166,56 @@ function file_save_upload($source, $validators = array(), $destination = FALSE,
       return FALSE;
   }
 
-  // Build the list of non-munged extensions.
-  // @todo: this should not be here. we need to figure out the right place.
-  $extensions = '';
-  foreach ($user->roles as $rid => $name) {
-    $extensions .= ' ' . variable_get("upload_extensions_$rid",
-    variable_get('upload_extensions_default', 'jpg jpeg gif png txt html doc xls pdf ppt pps odt ods odp'));
-  }
-
   // Begin building file object.
   $file = new stdClass();
   $file->uid      = $user->uid;
   $file->status   = 0;
-  $file->filename = file_munge_filename(trim(basename($_FILES['files']['name'][$source]), '.'), $extensions);
+  $file->filename = trim(basename($_FILES['files']['name'][$source]), '.');
   $file->uri      = $_FILES['files']['tmp_name'][$source];
   $file->filemime = file_get_mimetype($file->filename);
   $file->filesize = $_FILES['files']['size'][$source];
 
-  // Rename potentially executable files, to help prevent exploits.
-  if (preg_match('/\.(php|pl|py|cgi|asp|js)$/i', $file->filename) && (substr($file->filename, -4) != '.txt')) {
+  $extensions = '';
+  if (isset($validators['file_validate_extensions'])) {
+    if (isset($validators['file_validate_extensions'][0])) {
+      // Build the list of non-munged extensions if the caller provided them.
+      $extensions = $validators['file_validate_extensions'][0];
+    }
+    else {
+      // If 'file_validate_extensions' is set and the list is empty then the
+      // caller wants to allow any extension. In this case we have to remove the
+      // validator or else it will reject all extensions.
+      unset($validators['file_validate_extensions']);
+    }
+  }
+  else {
+    // No validator was provided, so add one using the default list.
+    // Build a default non-munged safe list for file_munge_filename().
+    $extensions = 'jpg jpeg gif png txt doc xls pdf ppt pps odt ods odp';
+    $validators['file_validate_extensions'] = array();
+    $validators['file_validate_extensions'][0] = $extensions;
+  }
+
+  if (!empty($extensions)) {
+    // Munge the filename to protect against possible malicious extension hiding
+    // within an unknown file type (ie: filename.html.foo).
+    $file->filename = file_munge_filename($file->filename, $extensions);
+  }
+
+  // Rename potentially executable files, to help prevent exploits (i.e. will
+  // rename filename.php.foo and filename.php to filename.php.foo.txt and
+  // filename.php.txt, respectively). Don't rename if 'allow_insecure_uploads'
+  // evaluates to TRUE.
+  if (!variable_get('allow_insecure_uploads', 0) && preg_match('/\.(php|pl|py|cgi|asp|js)(\.|$)/i', $file->filename) && (substr($file->filename, -4) != '.txt')) {
     $file->filemime = 'text/plain';
     $file->uri .= '.txt';
     $file->filename .= '.txt';
+    // The .txt extension may not be in the allowed list of extensions. We have
+    // to add it here or else the file upload will fail.
+    if (!empty($extensions)) {
+      $validators['file_validate_extensions'][0] .= ' txt';
+      drupal_set_message(t('For security reasons, your upload has been renamed to %filename.', array('%filename' => $file->filename)));
+    }
   }
 
   // If the destination is not provided, use the temporary directory.
diff --git a/modules/aggregator/aggregator.admin.inc b/modules/aggregator/aggregator.admin.inc
index 4deaf60859d..a2d9649171e 100644
--- a/modules/aggregator/aggregator.admin.inc
+++ b/modules/aggregator/aggregator.admin.inc
@@ -300,7 +300,8 @@ function aggregator_form_opml_validate($form, &$form_state) {
  */
 function aggregator_form_opml_submit($form, &$form_state) {
   $data = '';
-  if ($file = file_save_upload('upload')) {
+  $validators = array('file_validate_extensions' => array('opml xml'));
+  if ($file = file_save_upload('upload', $validators)) {
     $data = file_get_contents($file->uri);
   }
   else {
diff --git a/modules/locale/locale.admin.inc b/modules/locale/locale.admin.inc
index be25a35a09d..95edf3125d4 100644
--- a/modules/locale/locale.admin.inc
+++ b/modules/locale/locale.admin.inc
@@ -984,8 +984,9 @@ function locale_translate_import_form($form) {
  * Process the locale import form submission.
  */
 function locale_translate_import_form_submit($form, &$form_state) {
+  $validators = array('file_validate_extensions' => array('po'));
   // Ensure we have the file uploaded
-  if ($file = file_save_upload('file')) {
+  if ($file = file_save_upload('file', $validators)) {
 
     // Add language, if not yet supported
     drupal_static_reset('language_list');
diff --git a/modules/locale/locale.test b/modules/locale/locale.test
index ecc502d90fc..5b023e381ed 100644
--- a/modules/locale/locale.test
+++ b/modules/locale/locale.test
@@ -758,7 +758,7 @@ class LocaleImportFunctionalTest extends DrupalWebTestCase {
    *   Additional options to pass to the translation import form.
    */
   function importPoFile($contents, array $options = array()) {
-    $name = tempnam(file_directory_path('temporary'), "po_");
+    $name = tempnam(file_directory_path('temporary'), "po_") . '.po';
     file_put_contents($name, $contents);
     $options['files[file]'] = $name;
     $this->drupalPost('admin/config/regional/translate/import', $options, t('Import'));
@@ -905,7 +905,7 @@ class LocaleExportFunctionalTest extends DrupalWebTestCase {
   function testExportTranslation() {
     // First import some known translations.
     // This will also automatically enable the 'fr' language.
-    $name = tempnam(file_directory_path('temporary'), "po_");
+    $name = tempnam(file_directory_path('temporary'), "po_") . '.po';
     file_put_contents($name, $this->getPoFile());
     $this->drupalPost('admin/config/regional/translate/import', array(
       'langcode' => 'fr',
diff --git a/modules/simpletest/tests/file.test b/modules/simpletest/tests/file.test
index e501f57d09b..fe0ebc43119 100644
--- a/modules/simpletest/tests/file.test
+++ b/modules/simpletest/tests/file.test
@@ -537,12 +537,17 @@ class FileSaveUploadTest extends FileHookTestCase {
   /**
    * An image file path for uploading.
    */
-  var $image;
+  protected $image;
+
+  /**
+   * A PHP file path for upload security testing.
+   */
+  protected $phpfile;
 
   /**
    * The largest file id when the test starts.
    */
-  var $maxFidBefore;
+  protected $maxFidBefore;
 
   public static function getInfo() {
     return array(
@@ -558,14 +563,17 @@ class FileSaveUploadTest extends FileHookTestCase {
     $this->drupalLogin($account);
 
     $this->image = current($this->drupalGetTestFiles('image'));
-    $this->assertTrue(is_file($this->image->uri), t("The file we're going to upload exists."));
+    $this->assertTrue(is_file($this->image->uri), t("The image file we're going to upload exists."));
+
+    $this->phpfile = current($this->drupalGetTestFiles('php'));
+    $this->assertTrue(is_file($this->phpfile->uri), t("The PHP file we're going to upload exists."));
 
     $this->maxFidBefore = db_query('SELECT MAX(fid) AS fid FROM {file_managed}')->fetchField();
 
-    // Upload with replace to gurantee there's something there.
+    // Upload with replace to guarantee there's something there.
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
-      'files[file_test_upload]' => drupal_realpath($this->image->uri)
+      'files[file_test_upload]' => drupal_realpath($this->image->uri),
     );
     $this->drupalPost('file-test/upload', $edit, t('Submit'));
     $this->assertResponse(200, t('Received a 200 response for posted test file.'));
@@ -630,6 +638,158 @@ class FileSaveUploadTest extends FileHookTestCase {
     $this->assertFalse(file_load_multiple(), t('No files were loaded.'));
   }
 
+  /**
+   * Test extension handling.
+   */
+  function testHandleExtension() {
+    // The file being tested is a .gif which is in the default safe list
+    // of extensions to allow when the extension validator isn't used. This is
+    // implicitly tested at the testNormal() test. Here we tell
+    // file_save_upload() to only allow ".foo".
+    $extensions = 'foo';
+    $edit = array(
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload]' => drupal_realpath($this->image->uri),
+      'extensions' => $extensions,
+    );
+
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $message = t('Only files with the following extensions are allowed: ') . '<em class="placeholder">' . $extensions . '</em>';
+    $this->assertRaw($message, t('Can\'t upload a disallowed extension'));
+    $this->assertRaw(t('Epic upload FAIL!'), t('Found the failure message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate'));
+
+    // Reset the hook counters.
+    file_test_reset();
+
+    $extensions = 'foo gif';
+    // Now tell file_save_upload() to allow ".gif".
+    $edit = array(
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload]' => drupal_realpath($this->image->uri),
+      'extensions' => $extensions,
+    );
+
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $this->assertNoRaw(t('Only files with the following extensions are allowed:'), t('Can upload an allowed extension.'));
+    $this->assertRaw(t('You WIN!'), t('Found the success message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate', 'load', 'update'));
+
+    // Reset the hook counters.
+    file_test_reset();
+
+    // Now tell file_save_upload() to allow any extension.
+    $edit = array(
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload]' => drupal_realpath($this->image->uri),
+      'allow_all_extensions' => TRUE,
+    );
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $this->assertNoRaw(t('Only files with the following extensions are allowed:'), t('Can upload any extension.'));
+    $this->assertRaw(t('You WIN!'), t('Found the success message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate', 'load', 'update'));
+  }
+
+  /**
+   * Test dangerous file handling.
+   */
+  function testHandleDangerousFile() {
+    // Allow the .php extension and make sure it gets renamed to .txt for
+    // safety. Also check to make sure its MIME type was changed.
+    $edit = array(
+      'file_test_replace' => FILE_EXISTS_REPLACE,
+      'files[file_test_upload]' => drupal_realpath($this->phpfile->uri),
+      'is_image_file' => FALSE,
+      'extensions' => 'php',
+    );
+
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $message = t('For security reasons, your upload has been renamed to ') . '<em class="placeholder">' . $this->phpfile->filename . '.txt' . '</em>';
+    $this->assertRaw($message, t('Dangerous file was renamed.'));
+    $this->assertRaw(t('File MIME type is text/plain.'), t('Dangerous file\'s MIME type was changed.'));
+    $this->assertRaw(t('You WIN!'), t('Found the success message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate', 'insert'));
+
+    // Ensure dangerous files are not renamed when insecure uploads is TRUE.
+    // Turn on insecure uploads.
+    variable_set('allow_insecure_uploads', 1);
+    // Reset the hook counters.
+    file_test_reset();
+
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $this->assertNoRaw(t('For security reasons, your upload has been renamed'), t('Found no security message.'));
+    $this->assertRaw(t('File name is !filename', array('!filename' => $this->phpfile->filename)), t('Dangerous file was not renamed when insecure uploads is TRUE.'));
+    $this->assertRaw(t('You WIN!'), t('Found the success message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate', 'insert'));
+
+    // Turn off insecure uploads.
+    variable_set('allow_insecure_uploads', 0);
+  }
+
+  /**
+   * Test file munge handling.
+   */
+  function testHandleFileMunge() {
+    // Ensure insecure uploads are disabled for this test.
+    variable_set('allow_insecure_uploads', 0);
+    $this->image = file_move($this->image, $this->image->uri . '.foo.gif');
+
+    // Reset the hook counters to get rid of the 'move' we just called.
+    file_test_reset();
+
+    $extensions = 'gif';
+    $edit = array(
+      'files[file_test_upload]' => drupal_realpath($this->image->uri),
+      'extensions' => $extensions,
+    );
+
+    $munged_filename = $this->image->filename;
+    $munged_filename = substr($munged_filename, 0, strrpos($munged_filename, '.'));
+    $munged_filename .= '_.gif';
+
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $this->assertRaw(t('For security reasons, your upload has been renamed'), t('Found security message.'));
+    $this->assertRaw(t('File name is !filename', array('!filename' => $munged_filename)), t('File was successfully munged.'));
+    $this->assertRaw(t('You WIN!'), t('Found the success message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate', 'insert'));
+
+    // Ensure we don't munge files if we're allowing any extension.
+    // Reset the hook counters.
+    file_test_reset();
+
+    $edit = array(
+      'files[file_test_upload]' => drupal_realpath($this->image->uri),
+      'allow_all_extensions' => TRUE,
+    );
+
+    $this->drupalPost('file-test/upload', $edit, t('Submit'));
+    $this->assertResponse(200, t('Received a 200 response for posted test file.'));
+    $this->assertNoRaw(t('For security reasons, your upload has been renamed'), t('Found no security message.'));
+    $this->assertRaw(t('File name is !filename', array('!filename' => $this->image->filename)), t('File was not munged when allowing any extension.'));
+    $this->assertRaw(t('You WIN!'), t('Found the success message.'));
+
+    // Check that the correct hooks were called.
+    $this->assertFileHooksCalled(array('validate', 'insert'));
+  }
+
   /**
    * Test renaming when uploading over a file that already exists.
    */
diff --git a/modules/simpletest/tests/file_test.module b/modules/simpletest/tests/file_test.module
index 6d4d1f8c518..78b80f33da7 100644
--- a/modules/simpletest/tests/file_test.module
+++ b/modules/simpletest/tests/file_test.module
@@ -47,7 +47,7 @@ function file_test_stream_wrappers() {
 function _file_test_form($form, &$form_state) {
   $form['file_test_upload'] = array(
     '#type' => 'file',
-    '#title' => t('Upload an image'),
+    '#title' => t('Upload a file'),
   );
   $form['file_test_replace'] = array(
     '#type' => 'select',
@@ -61,9 +61,28 @@ function _file_test_form($form, &$form_state) {
   );
   $form['file_subdir'] = array(
     '#type' => 'textfield',
-    '#title' => 'Subdirectory for test image',
+    '#title' => t('Subdirectory for test file'),
     '#default_value' => '',
   );
+
+  $form['extensions'] = array(
+    '#type' => 'textfield',
+    '#title' => t('Allowed extensions.'),
+    '#default_value' => '',
+  );
+
+  $form['allow_all_extensions'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Allow all extensions?'),
+    '#default_value' => FALSE,
+  );
+
+  $form['is_image_file'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Is this an image file?'),
+    '#default_value' => TRUE,
+  );
+
   $form['submit'] = array(
     '#type' => 'submit',
     '#value' => t('Submit'),
@@ -75,7 +94,7 @@ function _file_test_form($form, &$form_state) {
  * Process the upload.
  */
 function _file_test_form_submit(&$form, &$form_state) {
-  // Process the upload and validate that it is an image. Note: we're using the
+  // Process the upload and perform validation. Note: we're using the
   // form value for the $replace parameter.
   if (!empty($form_state['values']['file_subdir'])) {
     $destination = 'temporary://' . $form_state['values']['file_subdir'];
@@ -84,10 +103,26 @@ function _file_test_form_submit(&$form, &$form_state) {
   else {
     $destination = FALSE;
   }
-  $file = file_save_upload('file_test_upload', array('file_validate_is_image' => array()), $destination, $form_state['values']['file_test_replace']);
+
+  // Setup validators.
+  $validators = array();
+  if ($form_state['values']['is_image_file']) {
+    $validators['file_validate_is_image'] = array();
+  }
+
+  if ($form_state['values']['allow_all_extensions']) {
+    $validators['file_validate_extensions'] = array();
+  }
+  else if (!empty($form_state['values']['extensions'])) {
+    $validators['file_validate_extensions'] = array($form_state['values']['extensions']);
+  }
+
+  $file = file_save_upload('file_test_upload', $validators, $destination, $form_state['values']['file_test_replace']);
   if ($file) {
     $form_state['values']['file_test_upload'] = $file;
     drupal_set_message(t('File @filepath was uploaded.', array('@filepath' => $file->uri)));
+    drupal_set_message(t('File name is @filename.', array('@filename' => $file->filename)));
+    drupal_set_message(t('File MIME type is @mimetype.', array('@mimetype' => $file->filemime)));
     drupal_set_message(t('You WIN!'));
   }
   elseif ($file === FALSE) {