Missing feature flag processing/check
With v10 EXT:form provided a feature toggle for the mimetype default definition which changed between the versions.
To get rid of the old default mime types some legacy mimetypes where provided for v10 (toggable via said feature flag), to be then deleted with v11.
The changed mimetypes patch:
To me it seems that the patch has to be reapplied. (it might got lost with the whole restructuring of the form yaml files)