=== modified file 'Mailman/Handlers/Scrubber.py'

--- Mailman/Handlers/Scrubber.py	2018-06-17 23:47:34 +0000

+++ Mailman/Handlers/Scrubber.py	2018-11-10 18:57:54 +0000

@@ -87,6 +87,9 @@

     all = guess_all_extensions(ctype, strict=False)

     if ext in all:

         return ext

+    if ctype.lower == 'application/octet-stream':

+        # For this type, all[0] is '.obj'. '.bin' is better.

+        return '.bin'

     return all and all[0]