Including Content-Transfer-Encoding header in RestSharp multi-part messages? - http-headers

Question

I am using RestSharp to upload multi-part messages (basically, form data with a file attached at the end). The receiving service seems to require that I specify the Content-Transfer-Encoding header on each part or I get a 400 error on the POST.
I can't find any way in RestSharp to set this, or to specify "headers" that are actually inside the body of the parts. What I need to produce is a payload that looks like this:
POST http://server/object/create HTTP/1.1
User-Agent: RestSharp/105.2.3.0
Content-Type: multipart/form-data; boundary=-----------------------------28947758029299
Host: server:8080
Content-Length: 540
-------------------------------28947758029299
Content-Type: application/xml;charset=US-ASCII
Content-Disposition: form-data; name="form"
Content-Transfer-Encoding: 8bit
<form>this is some form data in XML format</form>
-------------------------------28947758029299
Content-Disposition: form-data; name="1.eml"; filename="1.email"
Content-Type: application/octet-stream; charset=ISO-8859-1
Content-Transfer-Encoding: binary
This is arbitrary file content to attach to the form.
-------------------------------28947758029299--
So far, this is what I've got (I'm prototyping in PowerShell but the results are identical in C#):
$request = New-Object RestSharp.RestRequest("iss", [RestSharp.Method]::POST)
$request.AddParameter("application/xml;charset=UTF-8", $form, [RestSharp.ParameterType]::RequestBody) | Out-Null
$request.AddFile("att.txt", "C:\Development\att.txt", "application/octet-stream; charset=ISO-8859-1") | Out-Null
$response = $rest.Execute($request)
which does everything I need except the Content-Transfer-Encoding part.
Is this possible with RestSharp, or should I drop down to use the HTTP client directly?