=== 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]