Support recognizing JPEG files without JFIF or Exif markers

[mohamadmansourX]

BPO	44539
Nosy	@ambv, @fbidu, @mohamadmansourX
PRs	bpo-44539: Support recognizing JPEG files without JFIF or Exif markers #26964

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-07-20.18:57:30.296>
created_at = <Date 2021-06-30.08:12:46.419>
labels = ['type-feature', 'library', '3.11']
title = 'Support recognizing JPEG files without JFIF or Exif markers'
updated_at = <Date 2021-07-20.18:58:16.520>
user = 'https://github.com/mohamadmansourX'

bugs.python.org fields:

activity = <Date 2021-07-20.18:58:16.520>
actor = 'lukasz.langa'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-20.18:57:30.296>
closer = 'lukasz.langa'
components = ['Library (Lib)']
creation = <Date 2021-06-30.08:12:46.419>
creator = 'mohamadmansourx'
dependencies = []
files = []
hgrepos = []
issue_num = 44539
keywords = ['patch']
message_count = 4.0
messages = ['396771', '397891', '397899', '397900']
nosy_count = 3.0
nosy_names = ['lukasz.langa', 'fbidu', 'mohamadmansourx']
pr_nums = ['26964']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44539'
versions = ['Python 3.11']

[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.
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

[ambv]

When JFIF and Exif markers are removed, what we're left with is a "raw" JPEG file.

I added a raw equivalent of Lib/test/imghdrdata/python.jpg by running:

$ exiftool -all= python.jpg -o python-raw.jpg

Mohamad's patch correctly adds support for such files. In fact, browsing through how other languages do this, it seems like the FF D8 FF DB sequence is a relatively popular "magic sequence". It's even listed in this Wikipedia page:

https://en.wikipedia.org/wiki/List_of_file_signatures

[ambv]

New changeset 3b56b3b by Mohamad Mansour in branch 'main':
bpo-44539: Support recognizing JPEG files without JFIF or Exif markers (GH-26964)
3b56b3b

[ambv]

Thanks, Mohamad! ✨ 🍰 ✨