Missing type:object in multipart causes example to be in json, not file uploads

Q&A (please complete the following information)

• Swagger-UI version: "3.17.0"
• Swagger/OpenAPI version: OAS 3

Content & configuration

See: http://editor.swagger.io/?url=https://gist.githubusercontent.com/ponelat/eed475786cabcad61e861509c926967d/raw/ff02d25ddfe5080ddaea98189f0d28e37f4554e2/without-type-object.yaml

ie: a mutlipart/form object, that doesn't include type: object

post:
      summary: Create an item
      requestBody:
        content:
          multipart/form-data:
            schema:
              # type: object # <-- makes it work
              properties: # Request parts should NOT be rendered as JSON
                image:
                  type: string
                  format: binary

Describe the bug you're encountering

The body component creates a json example instead of a list of file upload buttons

To reproduce...

Steps to reproduce the behavior:

View a spec as above.

Expected behavior

Try it out form renders with file upload parameters

Screenshots

Actual

Expected

Fix/Notes

Bam: https://github.com/swagger-api/swagger-ui/blob/master/src/core/plugins/oas3/components/request-body.jsx#L32
Doesn't use inference (ie: if properties, then its an object )
Aternatively ( outside issue ) we make this a validation error.

Start in src/core/plugins/oas3/components/request-body.jsx at the referenced line and reproduce the behavior with the linked multipart/form-data example. The change is done when a schema with properties but no explicit type: object renders file upload controls in the Try it out form instead of a JSON example.