On Linux and the Gnome Desktop, Evince is the default application for viewing Adobe Acrobat (PDF) files. Recently, it started giving me the message “Unhandled MIME type: ‘application/x-extension-pdf'”. Here’s how I fixed it.
1. Edit $HOME/.local/share/mime/packages/Override.xml
2. Replace “application/x-extension-pdf” with “application/pdf”
3. Run “update-mime-database $HOME/.local/share/mime”
[Reportedly](http://www.mail-archive.com/evince-list@gnome.org/msg00485.html), a bug in the Nautilus file manager causes this behavior.