Following is how you could associate two file extensions with two different
filetypes, in file $HOME/vimfiles/filetype.vim:
---start---
if exists("did_load_filetypes")
finish
endif
augroup filetypedetect
au! BufNewFile,BufRead *.cls setf vb
au! BufNewFile,BufRead *.csv setf csv
augroup END
---end---
You should be able to adapt this (removing the csv line) for your purpose.
John