How to test this? The follwing happens for an attachment of an encrypted mail on gpg4win-5.0.0-beta357, Outlook LTSC Standard 2024 @ win10:

• Original filename with max length: long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.max.symmetric_encryption_gnupg_de.odt
• Both displayed and downloaded filename is the same as the original one
• Temporary filenames in C:\Users\g10\AppData\Local\Microsoft\Windows\INetCache\Content.Outlook are
  • long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.odt
  • long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots.in.it.long.filename.with.some.dots (002).odt

Sorry, I just found out, that windows caps the filename earlier than max length, so my former tests were invalid.

I retried with the filename 123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.123456789.1.odt and the temporary filename is now attachment.odt or e.g. attachment (002).odt.

Possible improvements:

• Use original filename on save (attachment.odt)
• Use original filename on open
  

But as those long filenames are an edge case, I think it's ok. In any case, the extraction of the extension does now work now as intended, so I set this to done.

Looks good to me on vsd-3.3.3-beta90.12 @ win10 (temporary filename is now attachment.odt or e.g. attachment (002).odt)