Some tools such as the command-line email client neomutt can launch graphical applications. In order to select which application for which file, mutt uses the mailcap system, provided by the mime-support package.

mailcap gets its default informations from two sources: some files installed by packages distributing the applications in either /usr/lib/mime/packages in mailcap format or in /usr/share/applications in FreeDesktop format. The Debian Policy specifies that the packages that provide informations in the FreeDesktop format refrain from repeat them in mailcap format (9.7.2).

The GIMP image editor declares its capacity to open PDF files in the file /usr/share/applications/gimp.desktop. GNOME's default PDF reader, Evince, declares this in /usr/share/applications/org.gnome.Evince.desktop. Desktop environments that follow the FreeDesktop standard have access to extra informations that give the priority to Evince. The mailcap system does not access them and gives the priority to alphabetic order. Therefore when one opens a PDF with mutt, it opens with GIMP, which is not convenient.

Fortunately, mailcap is easy to configure. In order to change the priority for one's personal account, one just has to copy the evince entry that is found in /etc/mailcap and place it into $HOME/.mailcap. For instance (but beware, it is simplistic):

grep evince /etc/mailcap >> $HOME/.mailcap

edited on feb. 28th to add $HOME to the example.