Steps to reproduce:

I have an email with an attachment, it ends with this:

--016_BL0PR0102MB3395C7121E50B0398B7E826AAA799BL0PR0102MB3395
Content-Type: application/pdf;
name="file.pdf"
Content-Description: file.pdf
Content-Disposition: attachment;
filename="file.pdf"; size=1903695;
creation-date="Tue, 16 May 2023 23:00:55 GMT";
modification-date="Tue, 16 May 2023 23:00:56 GMT"
Content-Transfer-Encoding: base64

Some base64 contents here as usual

--016_BL0PR0102MB3395C7121E50B0398B7E826AAA799BL0PR0102MB3395--

It all appears to be a normal multi-part email sent by MS exchange (if headers are to be believed).

Actual results:

Thunderbird didn't show that there is an attachment in this email

Expected results:

GMail shows an attachment just fine, I see it in email's source code, it should be visible in Thunderbird as well.

Judging by some other bugs I have found on a similar topic this is maybe related to https://bugzilla.mozilla.org/show_bug.cgi?id=505172 ?

Without a test case, yes probably dupe of bug 505172

I don't want to share a confidential email, but if there are any details about that are helpful I can provide them.
At very least I have a reproduction and it doesn't seem like it should be difficult to fix.

(In reply to Nazar Mokrynskyi from comment #3)

I don't want to share a confidential email, but if there are any details about that are helpful I can provide them.

If you can provide the basic source structure of mime parts in the message (even without their contents) so that their relationships can be examined, that might help to see if it's case of bug 505172 (wrt multipart/mixed|related|etc.). Maybe you can edit the source similar to what you did in comment 0. With more effort, perhaps you could even replace the sensitive contents with some other contents.

At very least I have a reproduction and it doesn't seem like it should be difficult to fix.

I wouldn't bet on that, but if you have an easy fix, you'll be welcome! :-)