How to fix the MIME-Type of PDF attachments in Thunderbird E-Mails

Recent versions of Mozilla Thunderbird set a wrong MIME-Type for PDF-files that are attached to an E-Mail. Thunderbird sets the MIME-Type to text/html instead of pdf/application. As a result the mail client of the receiver tries to display the PDF-file as plaintext and this leads most of the times to a system lock or crash of the mail client. I really don’t know why Mozilla did this, but I’m sure that they had their reasons, because the problem is very obvious and also easy to fix:

1) Open an terminal or use your file-browser for the next steps.
2) Open the hidden directory ~/.thunderbird located in your home-folder.
3) Open the *.default folder (* could be anything).
4) Open the file mimeTypes.rdf with an editor of your choice.
5) Remove the following line (near the end of the file) from mimeTypes.rdf:

<NC:fileExtensions>pdf</NC:fileExtensions> 

6) Restart Thunderbird!

You’re done. From now on the MIME-Type of attached PDFs will be set correctly and you should have no complaints from people about your E-Mails.