I've collected over the last year some 230 file extensions and manually curated their descriptions so that whenever I find a file extension, it becomes possible to give the end-user a slight idea about what the extension is about.
Most of my code nowadays is written in Java but there is interest in porting some of this information to web apps. So I have exported a JSON list that you are welcome to download and use in your projects.
The list is available on GitHub at this link.
One thing to keep in mind is that I'm looking at extensions from a software developer perspective. This means that when the same extension is used for different programs, I usually favor the programs related to programming.
The second thing is that I collect more information about file extensions than the info you find on this JSON list. For example, I populate for each extension the applicable programming languages. Here is an example for .h source code files. Other values include information if the data is plain binary or text readable, the category to which the extension belongs (archive, font, image, sourcecode, ..) and other meta data values that are useful for file filtering and processing.
If you need help or would like to suggest something to improve the list, just let me know.