I had a few things suggested to me, such as setting putting settings in my .vimrc, most of which would have forced filetype=sql on files that weren't sql, and would have been active outside of psql.
When you have a question what's the first thing you should do? RTFM. The answer was in the man page all along.
Tip: psql searches the environment variables PSQL_EDITOR,
EDITOR, and VISUAL (in that order) for an editor to use.
If all of them are unset, vi is used on Unix systems,
notepad.exe on Windows systems.
so how does this solve the problem? well the only program that uses PSQL_EDITOR is Postgres, and most editors allow you to specify options at startup. In the case of vim, if you want to have use filetype=sql for the default filetype for \edit all you have to do is
export PSQL_EDITOR='vim -c "set ft=sql"'
of course if you don't use vim or you want to set some different options you'll have to read the documentation for your editor, or operating system.