I develop the open source library django-email-extras which allows you to add email addresses and their associated PGP keys to the database, which will then encrypt those emails if they're sent to, including attachments.
When I originally developed it a few years ago, I think I was using Outlook with a PGP plugin, and decrypting attachments worked fine without any special handling. I've since had a report that with certain software, the attachments require an .asc extension in order to be correctly decrypted.
My questions is, is the .asc extension for encrypted attachments a standard that I should add handling for across the board, or is this something that's email client specific and therefore should be configurable per user?
PGP-MIME standard (RFC 3156) doesn't tell anything about attachment extensions. Probably, this is requirement/limitation of third-paty software which gets some file parameters from extension (.asc is usually used for armored PGP messages), and then feeds attachment to GnuPG.