Fix nuxeo-dropzone blob list display in create/edit/import layouts

Uploading blobs multiple times using a nuxeo-dropzone in a Document's create layout overwrites the displayed list of attached blobs with each successive upload; however all uploaded blobs are used when populating the bound schema property (e.g. "files:files") at document creation. The nuxeo-dropzone element should show all uploaded blobs at all times.

Steps to reproduce:

1. Edit the OOTB nuxeo-file-create-layout.html to include a nuxeo-dropzone bound to the files:files property:

   <nuxeo-dropzone value="{{document.properties.files:files}}" 
     multiple aria-labelledby="filesField" name="files" 
     value-key="file" message="Upload attachments" 
     drag-content-message="Drop your attachments" 
     required error-message="Attachment required">
   </nuxeo-dropzone>
   

1. Begin creating a new File document in Web UI
2. Click "Upload attachments" and select 2+ files to attach at the same time.
3. Click "Upload attachments" again and select a single additional file - observe that the displayed files from Step 3 are overwritten with the new one.
4. Create the File document with these attachments.

Expected behavior: the newly selected file(s) are appended to the list of uploaded blobs and are all visible prior to Document creation.

Actual behavior: the files displayed from Step 3 are overwritten with the most recently uploaded file(s), but the Document is attached with all uploaded blobs at creation.