set filetype in .vimrc

i have the following in .vimrc:
au BufNewFile,BufRead *diff*.txt set ft=diff_help
but then when i do
tabnew diff.txt
set ft
i get
filetype=help

why?

by the way, i have later on in .vimrc
filetype on
does this override the autocommand?

because from insdie the buffer when i set a help type file (*.txt) manually
to anything (for exapmle:'new') and then the command filetype, the filetype
doesn't changed

thanks
alex

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