In most cases, your computer or mobile device will recommend the appropriate software. However, there are times when it may not know the correct program to use. In those cases, it may provide a list of several programs, or attempt to open the file in the wrong program. If you are not sure what program you need, do an Internet search for the file’s extension (this is the 2-4 letters after the period in the filename). It may be necessary to download additional software to view the attachment.