There has probably been a time when someone has sent you a file, and when you tried to open it, you were unable to do so. There are a variety of reasons that this might happen. In most cases, it is because the file was created with a particular program and you don’t have that program installed on your computer.
NOTE: as I have advised many times in the past, if you don’t know ahead of time that the file you are opening is legitimate and not a virus or spyware, you should not try to open it.
In the past, you might forever wonder what was in that file. Most likely, you don’t want to purchase a program just to open that one file.
If you receive a file that you would like to see, but are unable to open, try this website: www.openwith.org. All you need to know is the file’s extension (those are the 3 digits that follow the filename). You can look up the file type in the alphabetical list, and the site will tell you what program(s) you can use to open that file. There are even direct links to download the program you want to use. In most cases, the downloads are free.