File Viewer: Implement styling within the iframe

[sondreb]

There are various attempts at putting some styling within the iframe of the file viewer, to make images render full width, but no success. So there are some unused code in the repo for this.

This task is to investigate and find a good solution to render content better within the iframe on the file viewer. This could be done by loading an HTML-template file instead of simply assigning the blob URL.