Some valid JPEG images are not detected by the imghdr lib.

Other valid jpeg image not detected.

patch for python 2.7

patch for python 3.X

patch for python 3.X

Working imghdr lib for python 2.7.X

Working imghdr lib for python 3.X

There seem to be unrelated changes in your patches, and we'll want tests as well before committing.  See also issue 16512 and the issues linked from it.  (I haven't reviewed to see if there is any overlap.)

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.

There is also #27121 reporting similar problem.

New patch for imghdr bug, including unittets. This patch works on python 3.x

Image used in the unit-tests of previous patch.
Add it under Lib/test/imghdrdata/python1.jpg

The issue16512 don't solve the problem, note how the patch it provide fails to detect all the valid JPEG images attached in this issue.

New patch for imghdr bug, including unittets. This patch works on python 2.7

imghdr is deprecated as per PEP 594, so there won't be further enhancements to it.