Symfony version(s) affected: looks like 4.2+

Description
Type error: Type error: Return value of Symfony\Component\Mime\MimeTypes::getMimeTypes() must be of the type array, string returned when passing custom map through constructor.

How to reproduce
$type =(new MimeTypes([ 'application/octet-stream' => ['rrd'] ]))->getMimeTypes('rrd')

Possible Solution
Add []'s around $mimeType in constructor