Default mimetype value must be application/octet-stream when uploading a file with an unknown mimetype

When uploading a file with an unknown mimetype in the platform, the default mimetype property value is set to '???' that breaks the webengine blob download api

<host>/nuxeo/site/api/v1/id/{docuid}/@blob/file:content

because we try to set this mimetype in the response header (see attached log).

[org.nuxeo.ecm.webengine.app.WebEngineExceptionMapper] Exception in JAX-RS processing
java.lang.IllegalArgumentException: Error parsing media type '???'