Updating file extraction for files#create action

When updating to Hydra 5.3 for Sufia, I noticed our installation was
sending the following params:

```
  {:files => ["","", <Actual File Object>]}
```

instead of the expected:

```
  {:files => [<Actual File Object>]}
```

I've updated how the file is extracted from the params[:file]. I have
also added the :unprocessable_entity status as that is a more customary
HTTP status on failure (for javascript requests sending 200 status when
there is an error may confuse the javascript client listening to the
request)