fix: add support for MPO encoded jpeg images.

[lovelock]

fix this issue: https://github.com/scanny/python-pptx/issues/787

As always the lib does not support .heic images, so I have to convert those iPhone images to .jpeg or .png and it goes well for the last two years. Yesterday I tried various methods, including ffmpeg, sips, imagemagick, pillow, and pyheif, but none could resolve the MPO issue. Finally, I attempted to modify the source code of python-pptx by adding an entry "MPO": "jpg" to the ext_map, and the problem was solved—the generated pptx file worked fine. So this isn't actually a compatibility issue; it was just the ext_map limiting the functionality.