In 19115-2 MX_DataFile was defined as having a fileFormat element with type MD_Format.
In 19115-3 MX_DataFile is defined in metadataTransfer.xsd as having a featureTypes element with type gco:GenericName_PropertyType.
Looked at this again... MX_DataFile is defined in mdt/metadataTransfer.xsd as an extension of AbstractMX_File (adds featureTypes to fileName, fileDescription, and fileType).
In 19115-2 MX_DataFile was defined as having a fileFormat element with type MD_Format. In 19115-3 MX_DataFile is defined in metadataTransfer.xsd as having a featureTypes element with type gco:GenericName_PropertyType.