fix flourish jpeg detection

Closes #523 (closed)

What does this MR do?

there is a bug in flourish, which causes not all images to be recognized leading to people not being able to login anymore https://github.com/flourishlib/flourish-classes/issues/198

bytes 3 and 4 are called the "application marker" APPn [2], where n stands for the least significant bits. In this case APP13, which stands for JPEG-HDR [3], a valid JPEG file.

as a fallback, we rely now on the php internal mime type detection.

• [1] https://gitlab.com/foodsharing-dev/foodsharing/blob/master/src/Lib/Flourish/fImage.php#L403
• [2] https://www.media.mit.edu/pia/Research/deepview/exif.html#ExifMarker
• [3] https://sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html

How confident are you it won't break things if deployed?

it feels do dirty, to change some code in the library...

Links to related issues

https://sentry.io/share/issue/47d28bf3d1eb41ba838cd4d23f6ee335/

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings