nuxeo-dropzone cannot be used to upload files on property other than files:files

Uploading files to a blob multivalued property is done using a `nuxeo-dropzone` element.
nuxeo-document-attachments is using this element to
attach files to files:files XPATH: https://github.com/nuxeo/nuxeo-web-ui/blob/master/elements/nuxeo-document-attachments/nuxeo-document-attachments.html

Issue is: even though `nuxeo-dropzone` defines a `xpath` property for defining where to attach files, this element is calling the `BlobHolder.AttachOnCurrentDocument` operation, which is not using xpath param and can only attach files to `files` schema: https://github.com/nuxeo/nuxeo-web-ui/blob/master/elements/nuxeo-dropzone/nuxeo-dropzone.html#L117

We should instead use the `Blob.AttachOnDocument` operation which defines a xpath param.