If renaming doesn't work, HandBrake is a popular tool for converting these files into different formats.
An file is a video container format developed by Apple, very similar to the more common MP4 format. It is often used for content downloaded from the iTunes Store and can sometimes include Digital Rights Management (DRM) protection to prevent unauthorized sharing. How to Open and Play the Video
If the file was purchased from iTunes and won't play on a non-Apple device, it is likely protected by FairPlay DRM, meaning it can only be viewed on authorized devices signed into your Apple ID.