Fix upload of multiple files with nuxeo-dropzone on 10.10

When using <nuxeo-dropzone> on 10.10 with the blob-list attribute (for a multivalued blob property) a user can only upload multiple files literally drag and dropping into the dropzone element. However, it should be possible to achieve the same result by clicking the element and selecting the files from the system dialog. This is currently not possible because the multiple attribute of the <input> element inside <nuxeo-dropzone> is not bound to anything on 10.10, while it is properly working on master.
(as discussed with Nuno Cunha)