Fix high CPU usage by 'soffice' & 'pdftohtml' process when viewing 'File' document whose attached file has a mime type "unrecognized" by the Web UI

When creating a File document, and then attaching e.g. a .mxf file, the Web UI will try to render a PDF preview, and background processes like soffice and pdftohtml will use 100% CPU.

How to reproduce:

• in the Web UI, create a File document
• attach e.g. a .mxf file (see sample file attached to this ticket)
• go back to the view layout

Actual result:
The view layout will try to render a PDF preview and background processes like soffice and pdftohtml will use 100% CPU (1 additional pdftohtml process every the user navigates to the document).
Expected result:
The view layout displays the file name of the attached file, a downlaod link, ... but no preview as it is a file with an "unrecognized" mime type.