FAX files generally use the TIFF format, meaning they are a kind of image file. In some cases, FAX files can also be used as templates with a specific program: Now Contact. In either case, the extension is used by and for faxes. In the case of Now Contact, the Fax file is used to store and copy layout information for faxes that need to be sent in bulk.
How can you open FAX files?
In most cases, whatever image management software you use should be able to open FAX files even without changing the file type or converting them to a different format.
As in most cases FAX files are images, they can be opened easily by renaming the file to be a .TIFF or .TIF. Almost any image viewer including the ones that come as standard with Windows and Mac machines can open that format. This solution is specific to FAX files and does not work for just any file type. Please remember that renaming a file is not the same as converting it, so if that is what you want to do, use an appropriate tool to do so.
What programs work with FAX files?
In addition to the Not Contact software mentioned above, programs like GIMP, XnView, InViewer and Adobe Photoshop all work with this file type, as does GFI FaxMaker.