EXIF allows 8 possible values for the Orientation field: 0°, 90°, 180°, 270°, and the same four for the mirror image of the original. The last four are not used often, but are supported to some degree by the browsers. (See http://www.daveperrett.com/articles/2012/07/28/exif-orientation-handling-is-a-ghetto/ for an overview of support.)

MediaWiki ignores mirrored rotations when generating thumbnails - the EXIF information is stripped, and the physical orientation is left unchanged. See [[commons:File:EXIF rotation 90 mirrored.jpg]] for an example (depending on the browser, you might need to open the image directly to see the correct position, or you might not see it at all - Chrome positions it correctly when viewed directly but not when viewed on a page).

Obviously this is Priority: No One Cares but filing it anyway for the sake of completeness.

• EXIF orientations: are in includes/media/ExifBitmapHandler.php#L209
• Access to flipping data via mediahandler
• Add transpose/transverse to imagemagick conversion in bitmaphandler
• https://github.com/wikimedia/operations-debs-python-thumbor-wikimedia/blob/555f99fd500a95e00778fa740ac08e41dc6ff896/wikimedia_thumbor/engine/imagemagick/imagemagick.py#L453

Note that it seems that thumbor does not depend on metadata info of MediaWiki in order to do its rotation, so it should be possible to fix this relatively easily in thumbor for WMF purposes, without having to add yet another metadata aspect to the mediahandler classes.