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I am using Ubuntu 14.04, and recently have a problem with Thunderbird: Some pdf files attached to emails are not treated as pdf files should be.

The attachment settings option for pdf's is set to automatically open pdf files with the document viewer. This works for some (most) pdf's files in attachments. However, for some files, a dialogue opens and asks what to do for the attachment, and suggests to open it with the video player. If I instead save the attachment to some folder, it is correctly recognized as a pdf and opened with the correct automatic program.

Where does this come from, and how can I fix it?

Edit: I checked the email source which states this:

Content-Type: video/x-flv;
 name="Erhebung.pdf"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="Erhebung.pdf"

apparently, the content type is wrongly recognized

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Apparently, this is a know issue with Thunderbird (https://bugzilla.mozilla.org/show_bug.cgi?id=503309). The main problem comes down to the fact that the sender can indeed specify "special" treatment for attachments that they include and Thunderbird then blindly follows the instructions in the email to open the PDF attachment as a video. What is even worse is that receiving such email and opening the attachment also adds the mime type to your MimeTypes.rdf and confuses Thunderbird further.

I know this doesn't help you much, but at least you now know what the issue is exactly and can try to manually work around it. This is also one more reason not to blindly open email attachments.

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    Thanks! From reading the bug description it is indeed even worse and might also use that for emails I send, right? What I could not figure out from the discussions is how I can manually work around the problem, for example, can I somehow manually reset what Thunderbird has "wrongly" learned? – user3825755 Oct 11 '16 at 13:57
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    Yes, you can edit your *.rdf file in ~/.thunderbird/[profile_name].default/mimeTypes.rdf look for mismatching/confusing filetypes and remove them. – kgizdov Oct 11 '16 at 14:03
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    Can I just remove the entire entry between <RDF:Description and </RDF:Description> ? or should I just remove strange <NC:fileExtensions> entries? – user3825755 Oct 11 '16 at 14:22
  • remove the entire entry including the start and end tags. – kgizdov Oct 11 '16 at 14:26
  • Thanks, I have done that now and couldn't detect any problems with any emails that I have saved. – user3825755 Oct 11 '16 at 14:32

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