Use our File Information Database.
Search for the file extension you are interested in in our database. You will find a list of programs that support files with a given extension.
Select and install one of the programs from the list. A file that you can't open should automatically be associated with your installed program.
In the vast majority of cases, you will already be able to open your file. The exceptions are when there are other problems (e.g. the file is damaged).