For the past year or so, we (and our users) have been fighting against e-mail filters which dislike the use of the .PRG extension that we use for program files. (.PRG is a little used alternative to .BAT; many e-mail filters are designed to prevent files which can be executed from being sent as attachments, thus blocking .BAT, .EXE, .COM, .PRG and a few others). In some cases, you can't even put the file into a .ZIP.
After some research, we have decided to standardize on the use of the extension .RPF, standing for RATS Program File. These will still be plain text---they just won't raise any flags with the mail filters. Future versions of RATS will have both .RPF and .PRG on the first item in the file type box on File-Open, and .RPF will open directly using RATS. (.PRG also has the problem that it's in use by several other programs). The other file types associated with RATS (.SRC for "source" files, .RGF for native graphics and .RAT for native data) seem to cause no problems and will be retained as is.
With an older version of RATS, the .RPF extension won't be on the File-Open list; however, you can just save as or rename as .PRG and you'll be able to use as before.