This is what our command line interface looks like right now:

Usage: pitivi [-p PROJECT_FILE] [-a] [MEDIA_FILE]...

Starts the video editor, optionally loading PROJECT_FILE. If no project is
given, pitivi creates a new project. Remaining arguments are treated as clips
to be imported into the project. If -a is specified, these clips will also be
added to the end of the project timeline.

Options:
  -h, --help            show this help message and exit
  -a, --add-to-timeline
                        Add each MEDIA_FILE to timeline after importing.


This means that to associate .xptv files with pitivi, we have to put "Exec=pitivi -p %f" in the .desktop file. Unfortunately, if we do that, then it's not possible to launch pitivi from the GNOME menu, as that would be like calling "pitivi -p" (without an argument to -p).

To solve this, we could change the command line to:

Usage: pitivi -i [-a] [MEDIA_FILE]...
       pitivi [PROJECT_FILE]

Starts the video editor, optionally loading PROJECT_FILE. If no project is
given, pitivi creates a new project.

Alternatively, when the -i option is used, command line arguments are treated as clips to be imported into the project. If -a is specified, these clips will also be added to the end of the project timeline.

Options:
  -h, --help            show this help message and exit
  -i, --import          Import each MEDIA_FILE in the project.
  -a, --add-to-timeline
                        Add each MEDIA_FILE to timeline after importing.

This way we could simply have "Exec=pitivi %f" in the .desktop file.