Fix EZP-23837: Impossible to set the value of a Media field

JIRA: https://jira.ez.no/browse/EZP-23837 (discovered while implementing the Media field edit view in PlatformUI https://jira.ez.no/browse/EZP-23813)

# Description

It's impossible to update the value of a Media field using the REST API. It seems like it's because unlike for BinaryFile and Image, the Media field type processor does not extend the BinaryInputProcessor (which takes care of creating a file on the disk with the data provided in the REST update struct).

Note: for now, this is a very rough patch just to let me work on EZP-23813 and to see which tests it breaks :)

# Tests

manual for now