An mp4 is a video file type, but the .mp4 in the file name is the file extension. Usually, these things should match, but it's possible for that not to be the case.

Normally, if you change a file’s extension in Windows, it doesn’t do anything positive. It just makes the file open in the ...

To change a file extension, right-click a file and click Rename. Add or change the extension at the end of the file name, after the period, and press the Enter key.

If you want to change all the TXT files in a directory to, let's say DOC, just open up a command prompt to that directory, and type:<BR>ren *.txt *.doc<BR><BR>ren is the command to rename.

To change the default file type in PowerPoint while saving a presentation, you need to open the PowerPoint Options panel first. Then, go to the Save tab and find the Save files in this format option.