An mpgp file extension is related to the MacPGP software for Mac. Probably obsolete format.
MPGP file extension - MacPGP data
What is mpgp file? How to open mpgp files?
File type specification:
The mpgp file extension is associated with the unofficial Mac port of PGP called MacPGP.
Most likely some old format without support from developer. No further information could be found.
This mpgp file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.