Hi @ff-sushild,
unfortunately the Faraday middleware for multipart doesn't currently allow to mix JSON with files.
If you send a multipart request, all the "body parts" (the parameters) will be missing the "content-type" header, so the server won't be able to automatically detect the content type.

I believe I've identified the issue in this line. The Part constructor accepts a forth parameter headers that should allow to do what you need, but we're not providing this at the moment.

Main reason for this is that mixing JSON and multipart is not really RESTful, so if you don't have the possibility to change the server implementation, a PR to fix this is more then welcome

However if you CAN change the server, you have other 2 options and I would strongly suggest to go for one of these:

1. Always force the server to parse the request body as JSON.
2. Send your files as JSON items and avoid the multipart/form-data request entirely (this works by encoding binary files in Base64). Unfortunately I only found a java example with a quick search, but I wouldn't be surprised if there's a gem for that.
3. Use a 2 phases approach (file and JSON body on separate requests) example.

Let me know what do you end up doing

Thanks @iMacTia, this is really very helpful to find the RCA and the solution. Will let you know where I end up with. Most probably will have to patch multipart for now and fixing the backend implementation for long term.

@ff-sushild sounds good! Glad I could help 👍
Totally agree with your approach

Hello, would you take a PR implementing the patch suggested here ? https://stackoverflow.com/questions/48679598/faraday-multipart-request-with-json-file-data?rq=1

class Faraday::Request::Multipart
  def create_multipart(env, params)
    boundary = env.request.boundary
    parts = process_params(params) do |key, value|
      if (JSON.parse(value) rescue false)
        Faraday::Parts::Part.new(boundary, key, value, 'Content-Type' => 'application/json')
      else
        Faraday::Parts::Part.new(boundary, key, value)
      end
    end
    parts << Faraday::Parts::EpiloguePart.new(boundary)

    body = Faraday::CompositeReadIO.new(parts)
    env.request_headers[Faraday::Env::ContentLength] = body.length.to_s
    return body
  end
end