Fix small image previews in the file viewer.

What does this MR do?

Fixes image previews being smaller than they should be.

Are there points in the code the reviewer needs to double check?

I don't think so.

Why was this MR needed?

Images were hard to view in the files view because they were limited to 50% of the file viewer's width.

What are the relevant issue numbers?

Mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/20505#note_13670085

Screenshots (if relevant)

Before:

After:

cc: @rymai