Allow to force-download attachments with a given mimetype even if they are added by a PR user

Steps to reproduce:

• Go to xwiki.properties
• open WEB-INF/xwiki.properties and go to the Attachment section
• uncomment the line with the default backlist and add to it a MIME-type , e.g.: application/pdf or image/png
• save the modifications and restart the server
• Go to in the wiki and open a page with an attachment pdf if you added application/pdf or a png image if you added image/png

Make sure you don't force the pdf files to be automatically downloaded from your browser's setting.

Expected results:

• The attachment is downloaded

Actual results

• The attachment is opened in browser