As GIMP and Audacity do, Kdenlive should register mimetypes and allow user to open a media file with Kdenlive, that is, Kdenlive not only expect project files as a parameter but also media files.

The expected behaviour with this: New project with those files in the project tree.

I think the -i option does what you are expecting. We may still need to register mime-types but, you can open files from the command line w/ Kdenlive.

Usage: kdenlive [Qt-options] [KDE-options] [options] [file]

An open source video editor.

Generic options:
--help Show help about options
--help-qt Show Qt specific options
--help-kde Show KDE specific options
--help-all Show all options
--author Show author information
-v, --version Show version information
--license Show license information
-- End of options

Options:
--mlt-path <path> Set the path for MLT environment

Arguments:
file Document to open
-i <clips> Comma separated list of clips to add
--database-mode Database mode. Login required
--database-cred <creds> Database credentials: username/password@database

The current syntax doesn't allow filenames with commas, and doesn't allow to make a .desktop launcher as a workaround to install a context-menu shortcut for this behaviour (cannot specify %F with the commas)