mtpaint displays an error message "Cannot open file"

Copied from: https://bugs.launchpad.net/ubuntu/+source/mtpaint/+bug/1714264

== Steps to reproduce ==
1. Using pcmanfm, right click on an image
2. Select "Open with mtpaint" or if that's not an option, select "Open with.." and select mtpaint from the "Graphics" section

== Expected behaviour ==
mtpaint opens the image

== Actual behaviour ==
mtpaint displays an error message "Cannot open file"

== Workaround ==
Edit /usr/share/applications/mtpaint.desktop and change the Exec line from:
Exec=mtpaint %U
to
Exec=mtpaint %f