FileInfo.com - The File Extensions Resource

.FAX File Extension

File Type	Fax Document
File Format	File
Common?	No
File Description	Fax image from a fax received or created with fax software; most often saved in a .TIFF format, which is recognized by most image viewing programs.
Program(s) that open fax files	Mac / Windows: Any picture viewer that supports TIFF files

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/fax