And these are PDF files, or mime encoded octet streams with no proper identifier as to what the data actually is. Unfortunately lots of mail originates from Microsoft mail clients and Microsoft are yet to grasp the concept that data types in MIME encoded mail are not set by file extensions. This is magnified by some particularly ignorant software developers using mostly Microsoft development tools and Office who have little idea what they are doing sending mail with scripts.

It may be that you need this add-on which makes Thunderbird use the same brain dead approach Microsoft does, the file extension not the content of the file https://freeshell.de//~kaosmos/index-en.html#openattach

Instructions http://chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html

I have been using Thunderbird for many years and never had this problem before. Why did this problem just start in the last month or so?

I have installed every update I can find and now most PDFs seem to open correctly from Thunderbird - but not all.

The PDFs that don't open correctly from Thunderbird do open OK if I save them and open them from a Windows folder.

Your "explanation" doesn't make sense. I have told Thunderbird and Windows to open files with a .pdf extension in Acrobat Reader. That is the program that should be used to open a file with the .pdf extension even if it isn't an Acrobat format file. Windows does it correctly, Thunderbird used to do it correctly, so what is screwed up now?

my explanation makes no sense because what I wrote does not fit into your understanding of files. So therefore it is nonsensical.

It is time to expand your horizons away from the Windows way of doing things and learn that MIME is how attachments travel in emails.

You have told Thunderbird to open portable document format files (identified as as an application/PDF in the MIME header of the mail. With adobe reader. That file could be called document.important for all Thunderbird cares. It is the mime type that is important.

You have told Windows to open files with an extension of PDF with adobe reader., Windows is brain dead with regard to what a file is and relies on the file extension to identify it's type.

Mozilla add-ons and Mozbackup are all zip files but windows fails to notice because someone changed the file extension. Likewise you mail attachments are most likely malformed, but I said that yesterday as well.

So to back up. I suggest you try the add-on I suggested yesterday.

I understand MIME - but that is irrelevant. You think Windows is brain-dead (so do I) - but that is irrelevant. You think you know a better way - but that is irrelevant.

The fact is that Windows is the most used operating system. Windows does use the file type to determine which program should be used. Any programmer who ignores this fact is a moron - in my opinion.

It doesn't matter whether it is "good" or "bad" - that's the way things work. Millions of people depend upon it to work that way every day.

You have failed to answer my question. Thunderbird once worked "correctly" with files having the .pdf file type. It does not now.

Why?

I suspect I know the answer. In the 40 years I have been designing and programming computers I have occasionally come across programming zealots who think they know a "better way" to do things, and they are determined to change the way everyone else does it. All they really accomplish is screwing things up for everyone.

Many programs do not follow the MIME protocol exactly - including Thunderbird. There are probably billions of .pdf files out there that are not "perfect" in someone's opinion. In cases where the MIME header doesn't tell what application should be used to open the file the .pdf file type does (more or less - there are many programs that can open PDFs).

Since you have cases where the user has specified that a file type be associated with a particular program, that file association should override anything in the MIME header. Thunderbird says the .pdf file type will be opened by Acrobat Reader (the default) but it isn't doing this.

That is a BUG and it should be fixed!

You need only to Google "Why does Thunderbird open PDF files in Word" to see that this bug is effecting many Mozilla program users.

If you are allowing your personal programming religion to screw things up for Mozilla users you should get yourself a broom and do something you are qualified for.

PRH PhD

The fact is that Windows is the most used operating system. Windows does use the file type to determine which program should be used. Any programmer who ignores this fact is a moron - in my opinion.

I will let the development team know you think they are morons. I am sure they will be pleased. Also be sure to include that in any bug report you file. As Thunderbird has no paid developers I am sure the volunteer doing the work will be overjoyed to correct something for someone that thinks they are a moron.

You have failed to answer my question. Thunderbird once worked "correctly" with files having the .pdf file type. It does not now. Why?

I have no idea. Has the mime type description changed by the sender. Perhaps they changed mail clients or went to a buggy web interface. "How long is a piece of string" is in this case an equivalent question. You refuse to accept the way Thunderbird works so your continued questions are really moot.

Since you have cases where the user has specified that a file type be associated with a particular program, that file association should override anything in the MIME header.

in your opinion. You have an option to change that so as far as I am concerned there is nothing more to say. Accept your floor needs attention.

Thunderbird says the .pdf file type will be opened by Acrobat Reader (the default) but it isn't doing this.

And I say if it is not correctly designated in the mime header then it is not a PDF file as far as Thunderbird is concerned. Regardless of you opinion. Regardless because it is simply not relevant.

That is a BUG and it should be fixed!

File a bug by all means. Those morons will be sure to jump at your suggestion.

If you are allowing your personal programming religion to screw things up for Mozilla users you should get yourself a broom and do something you are qualified for.

Interestingly I have no programming religion. What I see are facts. Thunderbird uses MIME types. Windows does not. Taking time to explain this to the ignorant is what support is about. Unfortunately some apparent are not capable of accepting facts.

I thank you for your high praise, and encouragement to continue volunteering here as I have done for years now. Perhaps I could step down here and you could take over. The degeneration is good for the sole and the pay is Zero.