The file extensions tell an operating system about the type of file so that the OS could select the right app or software to display the information in that file. The file extensions are written ...

Have you just downloaded a work PDF and suddenly found the file has the extension of a text file (TXT) or even no extension ...

Associating a file type with a program can be done in several ways, such as using the Open with dialog and selecting the Always use this app to open checkbox, or via Default Programs or Default Apps.