This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
The best thing to do is to try and see if there is any way to figure out
what -broad- type of file it is: word processor, picture, sound, or movie.
Tex-Edit Plus <http://hometown.aol.com/tombb/> will read
most of these out there though some will require Adobe Acrobat
Reader (.pdf), a commerical Word Processor such as
MS Word or WordPerfect, or a convertion utility like
Sound files, Pictures, and Movies
QuickTime <http://www.apple.com/quicktime/> is able to handle
the majority of the formats available on the Internet but
sometimes something else is needed for an uncommon format.
Sound App (Freeware, http://www.spies.com/~franke/SoundApp/)
is able to play Amiga MOD files and several other old formats
that Quicktime doesn't understand.
GraphicConverter (Shareware, $30-$35,<http://www.lemkesoft.de/>)
is one of the most powerful shareware graphic programs for the
Mac being able to open 100 different graphic formats (including
Ani, dl, gif, and fli/flc) and save in 60 of them.
In addition GraphicConverter has editing capablities
rivaling those seen in higher priced programs as well as being
able to create animated gifs.
More details on graphic formats in general can be found in the
QuickTime needs some help to handle the newer avi formats.
DivX Doctor II <http://doctor.3ivx.com/> and the related
3ivx Delta codec <http://www.3ivx.com/download/macos.html> are of
some assistant in this but even they do not let Quicktime handle
all avi formats.
Hopefully there is a document file that tells you what is needed
otherwise it is pretty much a lost cause.