In OS X Mavericks and later, QuickTime Player (version 10.0 and later) converts legacy media files that use certain older or third-party compression formats.
When you open such a file, a window with a progress bar shows that the file is converting. To stop converting, just close the window.
Media files that use many of these older formats will not be compatible with future macOS releases, so you should save the converted file before closing it. Conversion doesn't modify or replace the original file.
To preserve the quality of the original file, the converted file might be larger than the original.