I have a colleague who can successfully complete a multipart post of a CSV file with a Python implementation. Using the following C# code, I am able to produce the same request body as he does, but I still receive the "Unable to parse the request body as multipart form-data" error:   using (MultipartFormDataContent form = new MultipartFormDataContent()) { StringContent listIdContent = new StringContent(VendorListId); listIdContent.Headers.Remove("Content-Type"); form.Add(listIdContent, "\"list_ids\""); StringContent fileNameContent = new StringContent(fileName); fileNameContent.Headers.Remove("Content-Type"); form.Add(fileNameContent, "\"file\""); StreamContent streamContent = new StreamContent(System.IO.File.OpenRead(filePath)); ByteArrayContent fileContent = new ByteArrayContent(streamContent.ReadAsByteArrayAsync().Result); fileContent.Headers.Remove("Content-Type"); fileContent.Headers.Add("Content-Disposition", string.Format("form-data; name=\"data\"; filename=\"{0}\"", fileName)); form.Add(fileContent, "data", fileName); HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, requestUrl); request.Content = form; response = await client.SendAsync(request); responseJson = response.Content.ReadAsStringAsync().Result; }     Here is my request: Method: POST, RequestUri: 'https://api.cc.email/v3/activities/contacts_file_import', Version: 1.1, Content: System.Net.Http.MultipartFormDataContent, Headers: { Authorization: Bearer (followed by my access token) Content-Type: multipart/form-data; boundary="30473545-6c5e-44ff-8068-6dc192dc5241" Content-Length: 473 }   Here is my request body: --30473545-6c5e-44ff-8068-6dc192dc5241 Content-Disposition: form-data; name="list_ids" 3b1f9b40-8e75-11ec-a8dc-fa163e5bc304 --30473545-6c5e-44ff-8068-6dc192dc5241 Content-Disposition: form-data; name="file" example.csv --30473545-6c5e-44ff-8068-6dc192dc5241 Content-Disposition: form-data; name="data"; filename="example.csv" email,company_name ddipper@example.com,Test Company jacksbbq@example.com,Test Company --30473545-6c5e-44ff-8068-6dc192dc5241--   ... View more

Thank you. I will look at those resources while I am waiting for Aarron to finish troubleshooting. ... View more

Hi Aarron. Any updates on this? ... View more

Hi Aarron. Thanks for looking into this. Any updates on the troubleshooting? ... View more

I am also having trouble submitting my request as a multipart/form-data request body (I get the "Unable to parse the request body as multipart form-data" error). I understand that I need to include the "file" and "list_ids" properties in the body, but after several attempts I still cannot figure out how to do this in C#. Here is my code:     payload.Clear() .Append("{\"file\": \"") .Append(filePath) .Append("\", \"list_ids\": [\"") .Append(listId) .Append("\"]}"); requestUrl = "https://api.cc.email/v3//activities/contacts_file_import"; MultipartFormDataContent form = new MultipartFormDataContent("---Boundary"); form.Add(new StringContent(payload.ToString()), "payload"); Stream fs = System.IO.File.OpenRead(filePath); string fileName = Path.GetFileName(filePath); form.Add(new StreamContent(fs), fileName, fileName); response = await httpClient.PostAsync(requestUrl, form);     ... View more