Fix EZP-23390: Expose method to extract mimetype info on cluster files

Doubt @bdunogier : "One doubt, is legacy only taking first part of mime-type to store as datattype?
ref: https://github.com/ezsystems/ezpublish-kernel/blob/cluster_mimetype_EZP-23390/eZ/Publish/Core/IO/Handler/Legacy.php#L139"