What device are you referring to?
If you're referring to a Windows Phone running Windows Phone 8.1, then simply copy (or move*) that file into the Ringtones folder in your phone - that file should then appear in the Ringtones list when setting or choosing a Ringtone for any system event.
*- If you choose "move" over "copy", the file will be transferred (not duplicated) to the Ringtones folder and may disappear from your playlist.