pretty weird, I don't think we ever had this issue before... have a look at your es_systems.cfg file (look on the wiki if it's not clear, it's in /recalbox/share_init/system I think but there can also be a copy of it in /recalbox/share/system) and check if the person who sold/gave it to you didn't mess up with mame entry, you should find .zip among the accepted file extension for that system are your zip files at the root of your mame roms folder ?