FYI, the uptodate version of imghdr I maintain is here:
https://github.com/kovidgoyal/calibre/blob/master/src/calibre/utils/imghdr.py

It uses memoryview for performance and can also also read image sizes from file headers for jpeg, png, gif and jpeg2000. Note that is is only tested on python 2.7

I'm afraid I dont have the time to shepherd it through your review process, but feel free to take code from it if you want to. It is licensed GPLv3 but I am willing to re-license to another license if needed, as I am the sole contributor.