Rails Net::HTTP form data with custom headers - ruby-on-rails

Question

I am trying to send a file through form_data and set the Content-Type header as the content type of the file (image/png, image/jpeg, etc.) instead of multipart/form-data.
I have tried multiple ways, but here is my method:
def fetch(url, body, headers, limit, use_ssl, use_file_content_type)
raise HTTPRedirectLevelTooDeep, 'HTTP redirect too deep' if limit.zero?
headers[:'Content-Type'] = body['file'].content_type if use_file_content_type
uri = URI(url)
request = Net::HTTP::Post.new(url)
headers.each { |key, value| request[key.to_s] = value }
form_data = body.to_a
request.set_form form_data, 'multipart/form-data'
response = Net::HTTP.start(uri.hostname, uri.port, use_ssl: use_ssl) do |http|
http.request(request)
end
case response
when Net::HTTPSuccess then response
when Net::HTTPRedirection then fetch(response['location'], body, headers, limit - 1)
else
response.error!
end
end
This sends the body just as I want it, but does not set the Content Type header correctly (it uses multipart/form-data). I also tried setting the header after setting the form data:
request['Content-Type'] = body['file'].content_type right before http.request(request) but it sends Content-Type: application/x-www-form-urlencoded.
I also tried setting the header in the Net::HTTP::Post.new method, or moving everything after request = inside the Net::HTTP.start method, but it still does not work for me. I also tried using request.set_form form_data, body['file'].content_type, but that just says invalid enctype: image/jpeg
One thing that sent the headers correctly was replacing this:
form_data = body.to_a
request.set_form form_data, 'multipart/form-data'
with
request.body = body.to_json
But this does not send the file as the external API I'm using expects it.
Do you have any suggestions of how I could tackle this? Thanks!