Since MGNLUI-6390 / PR #1853, the UploadField performs mimetype parsing and validation thanks to the javax.activation.MimeType facility.

Since JDK9, this is no longer part of the standard JDK distribution, and the only Magnolia dependency to the old activation framework is transitively through mail module / javax.mail (now jakarta.mail, v2 change of namespace would break here). So we'd rather avoid it, in favor of a more general purpose library for this use case, most likely Guava or apache commons.