How to preserve the original file name as original_filename of a submitted file?

My users are submitting files that should be passed on to a third party.

The files are submitted as params[:files]:

= form_with :url => files_path, :method => :create, :multipart => true do
    = file_field_tag "files[]", :multiple => true

From my controller, they look like this:

[#<ActionDispatch::Http::UploadedFile:0x0000000103c1c678 @tempfile=#<Tempfile:/var/folders/k7/n71v0f_n2nb3pzyj38s5y9x80000gn/T/RackMultipart20220902-12314-in3y4h.txt>, @original_filename="test.txt", @content_type="text/plain", @headers="Content-Disposition: form-data; name=\"files[]\"; filename=\"test.txt\"\r\nContent-Type: text/plain\r\n">]

I go through the files like this:

params[:files].each do|file|
  HTTParty.post(
    "https://example.com/files",
    headers: {
      "Accept"        => "application/json",
      "Content-Type"  => "multipart/form-data"
    },
    body: {
      "file"          => File.open(file.tempfile)
    }
  )
end

Even though the files have a file.original_filename property, the one submitted in my POST request is the randomly temporary one. In this example RackMultipart20220902-12314-in3y4h.txt. Here is a trimmed log from my request:

<- "POST /files HTTP/1.1\r\nAccept: application/json\r\nContent-Type: multipart/form-data; boundary=------------------------dnh46jyEwRV85_DO\r\nAccept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3\r\nUser-Agent: Ruby\r\nConnection: close\r\nHost: example.com\r\nContent-Length: 433\r\n\r\n"
<- "--------------------------dnh46jyEwRV85_DO\r\nContent-Disposition: form-data; name=\"file\"; filename=\"RackMultipart20220902-12314-in3y4h.txt\"\r\nContent-Type: text/plain\r\n\r\nhello\r\n--------------------------dnh46jyEwRV85_DO\\r\n"
-> "HTTP/1.1 200 OK\r\n"

This means that the original file names are lost between me and the third party. How can I change that?