bpo-44539: Support recognizing JPEG files without JFIF or Exif markers

[mohamadmansourX]

Previous method to check JPG images was using the following command h[6:10] in (b'JFIF', b'Exif')
However, its not always the case as some might start with b'\xff\xd8\xff\xdb' header.
\xdb defining the Quantization Table.

Reference:
https://www.digicamsoft.com/itu/itu-t81-36.html
https://web.archive.org/web/20120403212223/http://class.ee.iastate.edu/ee528/Reading%20material/JPEG_File_Format.pdf

As an example, the attached image returned None from the imghdr.what(filename)

https://bugs.python.org/issue44539

[the-knights-who-say-ni]

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[fbidu]

Thanks for the PR, @mohamadmansourX. I was able to test this PR on a Linux Mint 20.1 x86_64 machine.

0. The supplied sample image when analyzed through the file command returns JPEG image data, baseline, precision 8, 500x332, components 3
1. Testing the current code for imghdr.what with the supplied sample image returns None.
2. After applying the patch, imghdr.what positively ids the image, returning jpeg

It would be nice to have a test case for this over Lib/test/test_imghdr.py. I see there are some tests there that involve only passing a hardcoded header and others that load images from Lib/test/imghdrdata. I think that at least a test with the harcoded header is welcome.

Also, please see the message about the CLA. There's also something wrong with the news entry but I couldn't figure that out

[merwok]

Could you remove all the commented lines?

[fbidu]

@merwok do you have any clues regarding the doc error? I tried removing the commented-out lines and running the sphinx toolchain locally but it all broke in the same way :/

[merwok]

It seems that some issue with the blurb filename hits a bug in blurb:

https://app.travis-ci.com/github/python/cpython/jobs/523615771

 File "/home/travis/virtualenv/python3.6.10/lib/python3.6/site-packages/blurb.py", line 486, in throw
    raise BlurbError(f("Error in {filename}:{line_number}:\n{s}"))
  File "/home/travis/virtualenv/python3.6.10/lib/python3.6/site-packages/blurb.py", line 138, in f
    return s.format_map(d)
KeyError: 'filename'

[merwok]

It’s known and fixed: python/core-workflow#386

Maybe blurb-it needs an update.