text: Title: Wrong Type for FileField based fields in POST type bodies
  Body: **Describe the bug**
  If I have a model with a models.FileField field, the type of this field is 'url' since the GET Request would include a url to the image, which is right. But when doing post requests, you don't have to provide an url but an file. 
  
  **To Reproduce**
  It would be most helpful to provide a small snippet to see how the bug was provoked.
  ```python
  class Bar(models.Model):
      Image = models.FileField(upload_to="events/", null=True, blank=True)
  
  ... Boilerplate with serializers.HyperlinkedModelSerializer Router
  ```
  
  The generated schema then contains 
  ```yml
  post:
        operationId: bars_create
        tags:
        - bars
        requestBody:
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Bar'
  ```
  and 
  `#/components/schemas/Bar` says:
  ```yml
  Bar:
        type: object
        properties:
          image:
            type: string
            format: uri
            nullable: true
  ```
  **Expected behavior**
  If i understand https://swagger.io/docs/specification/describing-request-body/file-upload/ correctly it should be `format: binary`