Fix rendition download to use proper reason/rendition in permission check

Extension point permissions of download service allows to restrict the downloads of a document's blob(s) based on a context provided to a javascript run() function.

When downloading renditions (e.g. mainBlob, xmlExport, ...), the context variable Rendition is always null, according to our documentation https://doc.nuxeo.com/910/nxdoc/file-download-security-policies/, it should contain the name of the rendition. Also, the context variable Reason is download, shouldn't it be rendition as also mentioned in our documentation?

How to reproduce:

• in Studio Modeler, create a new XML extension with this code:

     <extension target="org.nuxeo.ecm.core.io.download.DownloadService" point="permissions">
      <permission name="myperm">
        <script language="JavaScript">
          function run() {
            var allow = true;
            print("Document's UUID: " + (Document == null ? "null" : Document.getId()));
            print("Blob filename: " + (Blob == null ? "null" : Blob.getFilename()));
            print("XPath: " + XPath);
            print("CurrentUser's name: " + CurrentUser.getName());
            print("Reason: " + Reason);
            print("Rendition: " + Rendition);
            print("===");
            return allow;
          }
        </script>
      </permission>
    </extension>
  

• deploy the Studio project and start the Nuxeo Platform with command nuxeoctl console
• log in the JSF UI and get the XML export of a document (or use the REST API rendition endpoint as the Web UI does)

Expected result: the following output is produced in the terminal:

Document's UUID: null
Blob filename: document.xml
XPath: null
CurrentUser's name: Administrator
Reason: rendition
Rendition: xmlExport

Actual result: the following output is produced in the terminal:

Document's UUID: null
Blob filename: document.xml
XPath: null
CurrentUser's name: Administrator
Reason: download
Rendition: null

And in a use case like only allowing the preview of the Picture documents and blocking the download of the main blob in the Web UI, it is impossible to block the rendition mainBlob.

Here is another example: retrieving the picture view FullHD using the REST API endpoint rendition with URL api/v1/path/default-domain/workspaces/ws1/San_Francisco_Bay_Area.j/@rendition/FullHD, here is the log generated in the console:

Document's UUID: null
Blob filename: FullHD_San_Francisco_Bay_Area.jpg
XPath: null
CurrentUser's name: Administrator
Reason: download
Rendition: null