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)
Merge branch 'master' into sufia-for-curate-nd

* master:
  Updating file extraction for files#create action