FileFormat.com is described as 'is your File Format Guide. It gives comprehensive information, news, and discussions about popular File Extensions. Visitors can find Description as well as History of file formats with respect to their development phases'. There are three alternatives to FileFormat.com, not only websites but also apps for Windows and Windows Explorer. The best alternative is FileInfo.com, which is free. Other great sites and apps similar to FileFormat.com are OpenWith.org (Free) and File-Extensions.org (Free).
FileInfo.com contains a searchable database of thousands of file extensions with detailed information about the associated file types. You can use FileInfo.com to lookup information about unknown file types and find programs that open the files.