Fix previsualization of Office attached file to a File document

Currently, when visualizing an Office attached file to a File document, what one gets is the visualization of the main file of the File document, not the one of the annex file.

To reproduce:

• install Nuxeo 8.10 with JSF and configure LibreOffice 5
• (optionnally install hotfixes and upgrade JSF)
• create 2 (Libre)Office files with a different look: doc1 and doc2
• create a File document and attach doc1 as main file
• attach doc2 as supplementary file
• come back to the Summary tab, and previsualize doc2 (hit the eye button)
  ==> What you get is the visualization of doc1

(Notice that with text and Images, the previsualization of the attached pieces is as expected, so the issue seems to lie with (Libre)Office files only)

One can also notice in the File summary tab that the main file is available as a PDF but this does not happen for the annex file.