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$VIMRUNTIME/filetype.vim

$VIMRUNTIME/filetype.vim is used to detect file types.
It contains lines such as 
au BufNewFile,BufRead *.tyb,*.typ,*.tyc,*.pkb,*.pks	setf sql
$VIMRUNTIME/filetype.vim is run when :filetype on is executed.
TODO: is it also run when filetype plugin on is executed?
If it's not possible to detect tye file type by its name (mostly suffix), then Vim uses $VIMRUNTIME/scripts.vim to determine a file type.

Rough content of filetype.vim

The content of filetype.vim roughly looks like so: 
if exists("did_load_filetypes")
  finish
endif
let did_load_filetypes = 1


augroup filetypedetect

… 
… Stuff about ignored extensions etc.
…

au BufNewFile,BufRead $VIMRUNTIME/doc/*.txt   setf help
au BufNewFile,BufRead *.inp                   call dist#ft#Check_inp()
au BufNewFile,BufRead *.8th                   setf 8th

au BufNewFile,BufRead *.sql                   call dist#ft#SQL()
… etc. etc.

augroup END

" Source the user-specified filetype file, for backwards compatibility with Vim 5.x.
if exists("myfiletypefile") && filereadable(expand(myfiletypefile))
   execute "source " . myfiletypefile
endif


augroup filetypedetect

au BufNewFile,BufRead *
	\ if !did_filetype() && expand("<amatch>") !~ g:ft_ignore_pat
	\ | runtime! scripts.vim | endif
au StdinReadPost * if !did_filetype() | runtime! scripts.vim | endif


…
… Extra checks for when no filetype has been detected now.
…

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