Let pronterface.desktop open binary and ASCII STL, as well as Gcode

pronterface.desktop

@@ -4,8 +4,9 @@ Name=Pronterface
 GenericName=Printer Interface
 Comment=Controls your 3D printer
 Icon=/usr/share/pixmaps/P-face.ico
-Exec=/usr/bin/pronterface.py
+Exec=/usr/bin/pronterface.py %f
 Path=/usr/share/pronterface/
 StartupNotify=true
 Terminal=false
 Categories=GNOME;GTK;Utility;Graphics;
+MimeType=text/plain;application/octet-stream;application/sla;