Check to see if aolserver config has .doc as the word mime type. File storage makes use of ns_guess mime type or something. Aolserver must report it be word if its in the config file, otherwise it will default to text/plain.
If that doesn't help try executing 'select * from cr_mime_types;' manually and see if there is an entry for the 'doc' ending that points to 'application/msword'. If not, insert one.