I have seen this application/plain mentioned with .text suffix in some sources. Is it valid and has it been used anywhere? Or is this just a case where someone compiled a list and others just copied it?
It isn't a valid MIME type according to the IANA: http://www.iana.org/assignments/media-types/application/index.html
Sadly, that doesn't mean you won't encounter it in the real world. ;-)