apparently i cannot edit the post, so to add:

i don't think this is the intended behavior because of: the "delete messages when forwarded" flag, and also the fact that right now, a rule like this makes the spool fill up with the forwarded messages that fail to deliver to the original recipient, that eventually generate failure delivery reports that greatly confuses the original sender...

can we expect a quick fix on this?

This issue was resolved in our latest public release build 9340:

Fixed: Using a content filter to forward messages with specific attachment extensions to external addresses can create a delivery loop, preventing message delivery. 

What build are you currently running in this environment?

can confirm it is fixed, i was on build 9336