TextEdit keeps changing file extensions when I save Is there a way to prevent TextEdit from changing the file extension on a file that I open it with, or at least prompt me?  

Example:


Open somefilename.yyy
Save becomes somefilename.rtf


I have looked through TextEdit preferences without finding a setting.
Try with making the file to an Plain text file. It's under the Format tab (not in preferences).

Have you taken a look at this?

Apple support page