When linking program define the output name based on the OUTPUT_EXT before processing linking the file. This let's any hooks to use the final output file/name.

Omit the dot before the extension for OUTPUT_EXT (all Nordic and Teensy boards);

workspace_tools/targets.py

@@ -415,7 +415,7 @@ class K20D50M(Target):
         self.detect_code = ["0230"]
 
 class TEENSY3_1(Target):
-    OUTPUT_EXT = '.hex'
+    OUTPUT_EXT = 'hex'
     
     def __init__(self):
         Target.__init__(self)
@@ -742,7 +742,7 @@ class NRF51822(Target):
             'offset' : 0x14000
         }
     ]
-    OUTPUT_EXT = '.hex'
+    OUTPUT_EXT = 'hex'
     MERGE_SOFT_DEVICE = True
 
     def __init__(self):

workspace_tools/toolchains/__init__.py

@@ -633,6 +633,8 @@ class mbedToolchain:
 
     def link_program(self, r, tmp_path, name):
         ext = 'bin'
+        if hasattr(self.target, 'OUTPUT_EXT'):
+            ext = self.target.OUTPUT_EXT
 
         if hasattr(self.target, 'OUTPUT_NAMING'):
             self.var("binary_naming", self.target.OUTPUT_NAMING)
@@ -641,7 +643,6 @@ class mbedToolchain:
                 ext = ext[0:3]
 
         filename = name+'.'+ext
-
         elf = join(tmp_path, name + '.elf')
         bin = join(tmp_path, filename)
 
@@ -657,9 +658,6 @@ class mbedToolchain:
         self.var("compile_succeded", True)
         self.var("binary", filename)
 
-        if hasattr(self.target, 'OUTPUT_EXT'):
-            bin = bin.replace('.bin', self.target.OUTPUT_EXT)
-
         return bin
 
     def default_cmd(self, command):