There are all sorts of resources on the Internet to help you determine what program should open a particular file. Usually you do a search for the file extension, and you’ll find something telling you the program that made it or will open it. For instance, you can search for DOC and you find out that Microsoft Word is the most likely program that created and will open this file. If you search for AVI, you’ll find out it is a movie file, and there are dozens, if not hundreds, of programs that will play this file type.
I was recently informed about File-Extensions.org which is a site that tries to make this easier for you:
File-Extensions.org is a large list of the computer file extensions with detailed explanation of each file extension and the way it is used today.