You know what would rock? If common game file types were among the allowed types for PMs. Right now you can PM a game transcript but not a game itself.
Isn’t stuff like zip and rar allowed?
I would be nice if you could attach file types like .z8 and .gblorb.
Does it work if you change the extension to .txt and then have the recipient change it back? I think some people have posted .i7x files that way.
That wastes space on the forum. You should zip or rar it and then post it. I don’t see why anyone wouldn’t want to do that.
Unrelated but tangential enough for me not to feel comfortable making a new post - what about increasing the current maximum size (512kb) of attachments to, say, 2mbs? I just had to send over a 600kb zip for debugging purposes and I hit this limit which strikes me as ridiculously low…
I can up the size limit. With my online games no longer operating, I have extra space.
(edit)
I have upped the limit as requested, and added a few “document” extension types - .gblorb, .z8, .z5, .gam, and .i7x (although I don’t actually know what that last one is).
I’m going to go ahead and throw out there that phpBB has no way of validating the mime type of any of the new extensions, nor is it capable of verifying that they are what they claim to be. It will verify the extension and that’s it.
If you haven’t already, I’d suggest that you at least change the location of the attachment storage, preferably to something outside the web root. If not, anything besides the default. While the forum will do what it can to prevent anything unfortunate, you can’t be too safe. 
.i7x is an extension for Inform 7. Find lots of .i7x files at inform7.com/extensions/all-extensions/