Bad request by accent in the value of a property of the payload c# httpClient

Question

I am calling an api that uploads documents to an X repository using HttpClient.

I am emulating the call just like I do in postman using FORM-DATA. The problem is that I am serializing a request, that request has a property called filename and when the value of that property has a special character or an accent, the call immediately returns a Bad Request (400) but when that property goes without an accent, the service responds well OK (200)... what can I do?

public async Task Send(Book request)
{
    using (var client = new HttpClient())
    {
        string apiUrl = GetUrl();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, apiUrl);
        MemoryStream stream = new MemoryStream(Encoding.UTF8.GetBytes(request.documentBase64));

        var payload = new StringContent(JsonConvert.SerializeObject(request), Encoding.UTF8, "application/json");

        var requestContent = new MultipartFormDataContent()
        {
            { 
                new StreamContent(stream), "documentFormData", request.filename 
            },
            {
                 payload, "payloadRequest" 
            }
        };

        req.Content = requestContent;

        using (HttpResponseMessage response = await client.SendAsync(req))
        {
           //if request.filename = 'imagen_acción.png' gets BAD REQUEST(400)
           // but if request.filename = 'imagen_accion.png' gets OK(200)
        }
    }
}