Sorry, should add I'm running TB 102.4.2 64 bit in Linux Mint 21.2

I want to list attached files that are encrypted with PGP so that I can save them to the folder where I download other files types like PGP keys or PDF files.

What are you trying to achieve in the first place?

What prevents you from saving an attachment to a decrypted message by right-click it - Save As?

Nothing prevents me from doing that. But when an attachment has a file type of .pdf or .doc, that is something listed in the Content Type box, it will be downloaded to a specified location by just opening the email. I'd like to be able to do that with pgp or gpg files that come as attachments. Which means adding "any file".pgp or .gpg on the list of "Content Type" as I mentioned before. Its not a decrypted message. Its an encrypted attachment, attached to a standard text email. I'm not trying to decrypt the attachments, I want it to save to the same location as other email attachments like .pdf, .doc, or .jpg for example, which are listened in the Content Type box.

IANA register media types. Here is the full list. https://www.iana.org/assignments/media-types/media-types.xhtml

In that list I see quite a number of media types associated with encryption as described in RFC 3156 https://www.rfc-editor.org/rfc/rfc3156.html

I am guessing you mean media type pgp-encrypted. When you first try and open a file with that media type Thunderbird will prompt you for an application (helper application) or to save it to disk. Selecting to save that action will see it repeated.

What I see is many using mail appear to think file extensions have some relevance. They do not. Only the media type the sending application chose when it did the mime encoding is relevant in identifying the media type of the file. What I also observe is that many files are encrypted using incorrect media types from applications that are not well conceived. There is nothing to do with these poorly composed emails, except replace the program sending them.