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PNG has been updated for the first time in 22 years — new spec supports HDR and animation
(www.tomshardware.com)
This is a most excellent place for technology news and articles.
I'll bet you a shiny penny that's what it is. The backend recompresses things to some other format, probably a low bitrate JPEG, in order to save space and/or in case some joker uploads a 90 megabyte uncompressed TIFF image to use as a profile pic, or something.