The "mupdf.desktop" file provided in "mupdf/debian/" contains the line "Icon=mupdf.xpm" which makes applications such as geeqie report "Desktop file '/usr/share/applications/mupdf.desktop' should not include extension in Icon key: 'mupdf.xpm'". (I don't use a desktop/icons, so this is the only visible effect for me.) This is fixed by changing this line into "Icon=mupdf".