Je vois ça sur le wiki de Magic Lantern :

Images are saved in DCIM/1xxCANON/ after the following rules:

If intervalometer is OFF, silent pics are named after last picture/movie taken without this function (e.g. 12340001.422). You are limited to 10000 silent pictures for each noisy picture.
If intervalometer is ON, silent pics have names like 12345678.422. Tip: use File Numbering → Manual Reset from Canon menu to increase folder number (to sort them easier).
To convert a 422 image to JPEG on the PC, use one of the following programs:

422-jpg.py (command-line tool, runs on all platforms, you need to install Python, PIL and numpy).
422toimage (Windows only, source code available).
YUV422 Convertor (Windows only, closed source).