In OS X Mavericks and later, QuickTime Player (version 10) converts files that use certain older or third-party media formats. A window with a progress bar shows that the file is converting. To stop converting, just close the window.
Conversion doesn't modify or replace the original file, and you can save the converted file before closing it. To preserve the quality of the original file, the converted file might be larger than the original file.
QuickTime Player uses Apple ProRes to convert legacy formats used in video workflows, such as those encoded with the Animation codec. It uses H.264 to convert other older or third-party media formats.