supplemental:
I should point out that I don't *use* Thunderbird. I have it installed (the one from Fedora's preinstallation, not a different "daily" one), and can experiment with it, but I use Evolution. Out of all the email clients I've tried on Linux, it's the least-worst. Not the best recommendation, but that's how things evolved (pun intended)
Hunting around, I managed to find an email with a misidentified PDF file attachment (as application/octet-stream). Though I couldn't find one with a blank space in the name.
Evolution will open it when clicked on, just the same as a correctly identified one (as application/pdf), in my PDF reader. Or, perhaps I should say that my system (Mate spin of F36) does, Evolution has no obvious mimetype configuration options.
Thunderbird will open it when clicked on, within itself. Likewise for correctly identified ones. I only get open, save-as, detach, delete options, no open-with options, on the message reader.
If I go into the Thunderbird settings, and change PDF handling to an external reader, it works (correctly and incorrectly identified mimetyped files are opened externally).