Cannot render document with WebUI

On nightly:

1. create a testRendering workspace
2. create a File template with the attached template simple-nuxeo-tr.odt
3. create a File document
4. click the icon Render with template
5. select the previously created template
6. click Render (see rendering.png)
7. observe nothing is rendered and an error message (see unable.png) is displayed :

   Unable to render document: ''.
   

Expected behavior: a rendered file should be downloaded.
Note: this works fine with JSF UI.
Workaround: setting the rendering format (in JSF because the option is not available in Web UI) allows the rendering to be performed.