Fix the display, preview and download of custom BlobList properties

Steps to reproduce:

1. deploy the attached jar to nxserver/plugins
2. create a document of type WEBUI-1213
3. on the Summary view, at the bottom, upload an attachment
4. the page is refreshed
5. observe that a line is added before the "Upload Attachment zone" with icons => it confirms that the file was correctly uploaded and attached to the document

BUGS :

• the name and the size of the attached files are missing
• click on the Download icon => it fails because of a wrong URL : http://localhost:8080/nuxeo/ui/undefined
• click on the Preview icon => it fails with a server side error "java.lang.ClassCastException: class java.util.HashMap cannot be cast to class org.nuxeo.ecm.core.api.Blob"

About the preview, I compared with the URL generated for files:files :

• files:files => /nuxeo/nxfile/default/239f34ab-de9c-4841-9ea7-64f2c4c6fb4d/files:files/0/file/
• custom blobs => /nuxeo/site/api/v1/repo/default/id/faa58af4-b90e-4792-83d9-e95b0ce5db63/@blob/file_schema:attachments/0/@preview/