Thunderbird suddenly opens PDF attachments with ImageMagick

I am running Ubuntu 12.04 and use Thunderbird as my email client. People send me PDF-files all the time, and clicking on them used to open them in acroread. Since a few days, if I click on an attachment, it opens them with ImageMagick! I have no idea how this happened, I did not mess with PDF settings recently and I did not even know I had ImageMagick installed.

I checked under Edit->Preferences->Attachments, where is says 'Use Adobe Reader 9 (default)`. When right-clicking on a PDF file in Nautilus and doing Properties->Open With, it again lists 'Adobe Reader' as default and only 'Document Viewer' as an alternative option. I also cannot find ImageMagick anywhere in mimeTypes.rdf. Anybody knows what is going on or what to investigate further?

Solution 1:

I think I solved it myself: the problem was only occurring for PDFs sent by a certain colleague. Looking at the raw source of his mail, it says Content-Type: image/pdf, while for mails from other people it says Content-Type: application/pdf;. Searching around, it seems that there are many bugs related to Thunderbird's handling of mimetypes, see e.g. here.