Description:
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I decided to try PHP 7.3 today, and some things broke because both mime_content_type and finfo_file (with FILEINFO_MIME) were returning application/octet-stream when they shouldn't.

In the same system, with no changes,

file -i /path/to/the/same/file 

returned the correct MIME information for the file.

Downgrading to PHP 7.2.13 in the same system, with no other changes, same php.ini file parameters, fixed the problem. /etc/magic.mime has been empty throughout the whole test.

Test script:
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<?php
echo mime_content_type("/full/path/to/examplevideo.mkv");


Expected result:
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video/x-matroska; charset=binary

Actual result:
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application/octet-stream