Not just those. Files are just a method of storing digital data, so it’s not just those four. You can have files storing databases, software (think exe, AppImage, deb, rpm, etc.), design files, projects, and more!

And file extensions are a method of telling different programs how to handle different files, since the data is formatted a bit differently. For instance, a “.txt” file is stored in plain text, while an executable file is compiled code that needs to be run.

For your example, I would like to note that you are comparing a plain text file type to a rich text file type. Plain text file types, like .txt, .md (Markdown), and the different code files (like .json, .py, .rs, etc.), can be viewed and edited with a simple Notepad-style text editor. The data is stored, as the name suggests, in plain text. In comparison, rich text file types, like .odt and .docx, encode additional data like fonts, styles, images, animations, etc., and require a rich text processor (like LibreOffice, MS Office, etc.) to read them. You can’t view them through a notepad-style application, for example.

And for images, video, and audio, you have it take into account compression, codecs, that sort of thing. You might have heard that a PNG can store transparent images and is a lossless format while a JPEG cannot and is a lossy format. “Lossless” means that, after compression, no data has been removed (or “lost”), while “lossy” means that some data is removed after compression. For audio, MP3s are lossy while WAV files are lossless. You might have also heard of “raw” photos and “raw” videos, those mean that the data is directly from the camera in its original quality.

For most file types, you can’t just change the extension to convert them, as the data stored is arranged differently! This is why renaming a .txt file into a .odt will not be a valid rich text document, for example.