Can you create thumbnails for movie extras?

Is there a way I can save a JPG or image file so that it is picked up and used as a thumbnail for a movie extra?

For instance if I have an extra named [funny-deletedscene.mkv] I have tried using a thumbnail named [funny-deletedscene.jpg] but Plex doesnt seem to pick it up?

Any help would be awesome !

Thanks

Jason

Unfortunately, you cannot set a poster for a local movie extra.

Thanks Otto for the response