[Desktop Entry]
Type=Application
Name=Ginga
Comment=The Ginga astronomical FITS file viewer
Exec=ginga
Categories=Education;Science;Astronomy;
MimeType=image/x-fits;

[Desktop Entry]
Type=Application
Name=Ginga
Comment=The Ginga astronomical FITS file viewer
Exec=ginga %F
Categories=Education;Science;Astronomy;
MimeType=image/x-fits;

Add file name to exec
Add file name to exec 

This allows to use the desktop file to handle the file types that are actually specified by the mime tye.