support file uploads

[domanchi]

Summary

In the petstore example, /pet/{petId}/uploadImage accepts a file in the formData.

/pet/{petId}/uploadImage:
  post:
    parameters:
    - name: "file"
      in: "formData"
      description: "file to upload"
      required: false
      type: "file"

We don't currently support this. Maybe we should?