This is a follow-up from MDL-63399.

While working on that improvement I noticed that in most cases it's not possible to validate the file extension using php finfo class:

$finfo = new finfo(FILEINFO_EXTENSION);

$actualextension = $finfo->buffer($filecontent); 

In most xml and text files in moodle the actual extension will return '???' causing Behat to fail:

The file downloaded should have been a txt file, but got ??? instead. (Behat\Mink\Exception\ExpectationException)