add filetype comment for everyfile

I saw that most file has a comment at the bottom of file.
for example in ~/.muttrc
{{
set folder=~/.mutt/mails
# vim: ft=muttrc
}}
in this case, the last line, "# vim: ft=muttrc" is a filetype comment for file
muttrc.
I hope to add a function that let vim automatically add this last comment line
to every file in my ~/.vimrc .
Thanks in advance.

--
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I believe myself cause I can.