RAML Doc generator loses content type for file types

### Context

When I have a POST body of type [file](https://github.com/raml-org/raml-spec/blob/master/versions/raml-10/raml-10.md#file):

```
post:
displayName: Upload an image
queryParameters:
filename:
required: false
type: string
(builtinType): string
headers:
Content-Type:
type: string
(builtinType): string
required: true
body:
type: file
fileTypes:
- "image/jpeg"
- "image/png"
- "image/gif"
```
### Current behavior

The RAML Doc generator turns it to `application/json`:

```
body:
application/json:
fileTypes:
- "image/jpeg"
- "image/png"
- "image/gif"
type: file
(builtinType): file
```
### Expected behavior

The type stays as it was:

```
body:
type: file
fileTypes:
- "image/jpeg"
- "image/png"
- "image/gif"
```