I edited a .desktop (mtPaint) file to add missing mime types it can handle:
MimeType=image/x-targa;image/x-tga;image/x-pcx;image/gif;image/png;image/x-xpixmap;image/x-xpm;image/tiff;image/tiff-fx;image/x-ms-bmp;image/jpeg
but I don't get File to recognize this App when "Open with" on a file of that type. I tried logoff & login.