Fix regression brought by NXS-5019

NXS-5019 has changed how the filename property was read on a Blob field. It was done to follow the deprecation of the filename element in the "content" field (defined in core-types.xsd)

However the code which is not generated is incorrect because filename does not exist on the "content" field: see https://github.com/nuxeo/nuxeo/blob/master/nuxeo-core/nuxeo-core/src/main/resources/schema/core-types.xsd

The property name must be used instead like in https://github.com/nuxeo/nuxeo/blob/master/nuxeo-dm/nuxeo-platform-webapp-core/src/main/resources/OSGI-INF/layouts-contrib.xml#L436