If QuickTime Player converts legacy media files before playing them
You might see a progress bar showing that your file is converting.
In OS X Mavericks through macOS Mojave, 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.
Save the converted file
macOS Mojave is the final version of macOS to convert many of these older formats, so you should save the converted file in its compatible format 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.