Read a custom file extension

Question

Hi, I have a friend who is working on their end project with C #. It tells me that it works to code a software of recording and reading of piece of music through a file of extensions customize. My question is how I can read a custom extension file with the program. For example if I write a text file the program will save it with the .dev extension and then it can read it. Is there an example project or source code like that?