cubicweb-file #1371 add an another tab for the preview of a file [open]

The defaut rendering is not always pertinent and can move the download link to the bottom of the page.

I'd like to have a first pane with the file information (size, type, ...) and a second one with a possible preview (text conversion for exemple).

done in<not specified>
load left0.500
closed by<not specified>