Currently, the file preview makes very bad use of screen space:
It's hard to see anything in this window, even with small documents.
The value of having a "preview" per se is negated by the fact that still the entire file needs to be downloaded, so why not just display the file using the entire screen?
(at that point, the question comes up whether the UI elements are even needed or whether the file could just be downloaded and displayed with "Content-Disposition: inline")