MEP files are actually not video files, but projects from older versions of a tool called Movavi Video Editor. This means that standard multimedia converters won't be able to convert these files to some standard multimedia format (like QuickTime), but find older version of Movavi and use that to export your video.
Latest version of Movavi no longer supports .mep files (just .mepj), so if you struggle with your old .mep files, it would be best to contact their support.
Updated: July 21, 2023