Unable to send CSV data as part of payload (Playwright API with C#)

Question

I am using Playwright with C# and I need to send a POST request. Since I need to include a CSV data file in the payload, I am using multipart encoding, but have not had much luck so far. The CSV file lives in a folder in my VS project called TestData.

I am pretty sure the issue is with the Buffer. I have gotten a 200 response with no CSV data, so at least I know the API is working. Currently I am getting a 400 response with this code:

 public async Task<string> CreateNewSimulation()
        {
            await CreateSimulatorRequestContext();
            var csvFilePath = @"TestData\VT_Gateway.csv";
            var csvFileContent = File.ReadAllText(csvFilePath);
            var csvFileName = Path.GetFileName(csvFilePath);
            var file = new FilePayload()
            {
                Name = csvFileName,
                MimeType = "text/csv",
                Buffer = System.Text.Encoding.UTF8.GetBytes(csvFileContent)
            };
            var multipart = Request.CreateFormData();
            multipart.Set("files", file);
            multipart.Set("ReplayDelayMilliseconds", 3000);
            multipart.Set("Limit", 142);
            var simResponse = await Request.PostAsync("/api/devicesimulator/DeviceSimulator", new() { Multipart = multipart });
            NEW_SIM_MSG = simResponse.ToString();
            return NEW_SIM_MSG;
        }

These are the headers I am sending with the request:

 var headers = new Dictionary<string, string>
            {
                { "Content-Type", "multipart/form-data" },
                { "Authorization", "Bearer " + ACCESS_TOKEN },
                { "Ocp-Apim-Subscription-Key", OCP_KEY }
            };