What you are actually practicing
The few letters after the dot in a filename tell you what the file contains before you open it. It is a small piece of information literacy, and it saves a lot of guessing when a folder is full of unfamiliar names.
It also makes the file system slightly less mysterious. Once you know that .docx is a word processor document and .xlsx is a spreadsheet, you can usually work out what a file is from its name alone.
How the game works
You are shown an extension and four possible descriptions. Tap the correct one. A wrong pick shows the answer immediately.
Easy covers the extensions you meet every day. Medium adds the developer and system ones. Hard uses the full list.
The links worth knowing
Images: .jpg and .png are both image files, and .svg is a scalable vector image, which means it stays sharp at any size. .gif is for short looping animations.
Audio and video: .mp3 is an audio file, .wav is an uncompressed audio file, and .mp4 is a video file.
Documents and data: .pdf is a document file, .docx is a word processor document, .xlsx is a spreadsheet, and .csv is a plain table of data.
The web: .html is a web page, .css is a stylesheet, and .js is a script file. Those three are what the browser uses to draw every page you visit.
Archives and systems: .zip is a compressed archive, .exe is a Windows program, .iso is a disc image, .ttf is a font file, and .sql is a database query file.
How to get faster
Group them by what you would do with the file. Ones you open and read, ones you watch or listen to, ones you compress to save space, and ones a program reads rather than a person. The groups are obvious once you sort them, and most of the list falls into place.
A note on the overlap
Several extensions describe a format rather than a purpose. .html and .css are both plain text files that you could open in a text editor, and .pdf is a container that can hold text, images or both. The extension tells you the convention, not the contents.