Add support for 'multipart/form-data' file upload

I'm attempting to describe file upload, according to swagger docs for file-upload we should define the file as type string with the format of binary.

so my openapi for this request looks like:

      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                contractId:
                  type: string
                  example: 270A2087-6335-455E-B32D-350C00FFEEB7
                organisationId:
                  description: Your ID for this participant
                  example: AAA123
                  type: string
                resourceId:
                  type: string
                  example: 49E2797A-A62E-4042-9587-2CC0A1DBC679
                file:
                  type: string
                  format: binary

However by the time this gets to the committee middleware rack has exploded the file to a hash

file: {
  :filename=>"i_am_a_file.csv", 
  :type=>"text/plain", 
  :name=>"file", 
  :tempfile=>#<Tempfile:/tmp/RackMultipart20200306-13-1rx39tl.csv>, 
  :head=>"Content-Disposition: form-data; name=\"file\"; filename=\"i_am_a_file.csv\"\r\nContent-Type: text/plain\r\nContent-Length: 81\r\n"
}

so we end up with an error like:

#/paths/~1api~1v1~1telemetry_data_files/post/requestBody/content/multipart~1form-data/schema/properties/file expected string, but received Hash:

I was looking at test/request_unpacker_test.rb but I do not see how to handle this situation. What am I missing?

Start with test/request_unpacker_test.rb and trace how the multipart/form-data request body is unpacked before schema validation. Reproduce the OpenAPI example with Rack's file hash, then determine the expected representation for the file field. Done means the described upload no longer fails with an expected-string-but-received-Hash error.