Yes the Alwil team do monitor and participate in these forums, filtering on extension alone isn't common and is stupid, if just changing the extension gets around the restriction if not backed up by a scan is an absolute waste if everyones time.
Strange that MS didn't include .doc or .xls as potentially dangerous if they are going to say .tmp is, they are hardly going to restrict every MS Office file type so the double standard is amazing.
Microsoft Outlook Express, Microsoft Windows Messenger, Microsoft MSN Messenger, and Microsoft Internet Explorer use the Attachment Manager to handle e-mail attachments and Internet downloads.
So this is probably coming from Outlook Express and you can disable this restriction, Tools, Options, Security, untick Do not allow attachments ......... that is enabled by default, see image.
With this unticked hopefully you shouldn't suffer from any restriction.