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:

![Screen_Shot_2016-08-08_at_10.35.40_AM](/uploads/69d885d1c8bb6296f5ac61aab4a4bf91/Screen_Shot_2016-08-08_at_10.35.40_AM.png)

After:

![Screen_Shot_2016-08-08_at_10.35.50_AM](/uploads/b78ed41a781228d9f170ed559a1fda25/Screen_Shot_2016-08-08_at_10.35.50_AM.png)

cc: @rymai 

See merge request !5721