Sending Fileupload from HTTPClient to dotnet core Application

i have a Server Application using dotnet core Controllers. There I have this Controller:

        [HttpPost]
        [RequestSizeLimit(100_000)]
        public async Task<IActionResult> xxx([FromForm] string name, [FromForm] string description, [FromForm] bool isprivate, [FromForm] IFormFile icon)
{
   ...
}

When i send the request from my Frontend in the Browser, it works fine, generating this request:

Frame 103: 2148 bytes on wire (17184 bits), 2148 bytes captured (17184 bits) on interface \Device\NPF_Loopback, id 2
Null/Loopback
Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
Transmission Control Protocol, Src Port: 51141, Dst Port: 8001, Seq: 4701, Ack: 1792, Len: 2064
Hypertext Transfer Protocol
    POST /api/xxx/xxxHTTP/1.1\r\n
    Host: localhost:8001\r\n
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:76.0) Gecko/20100101 Firefox/76.0\r\n
    Accept: application/json, text/plain, */*\r\n
    Accept-Language: de,en-US;q=0.7,en;q=0.3\r\n
    Accept-Encoding: gzip, deflate\r\n
    Content-Type: multipart/form-data; boundary=---------------------------20261806215495369501698203786\r\n
    Content-Length: 1127\r\n
    Origin: http://localhost:8001\r\n
    DNT: 1\r\n
    Connection: keep-alive\r\n
    Referer: http://localhost:8001/xxx\r\n
     [truncated]Cookie: .AspNetCore.Cookies=CfDJ8M_ELI8knopFkQ6zEqeCkgwDf43UGRqyyzwLWbKrkSuSRcmzqAPIcsi94bVzQFVWJB0NOGVRZVgzm4n7sf-0ox3SPK1ChgaqQuPRTogxdWzbJZPo1DCCi1TMI04RH4tmMj8rgv8EwDko5s1S2BNw0Owj8qL_-loCOz2VYU7wG3lVl5TIVavb46_nV6_Zr6MVdz
    \r\n
    [Full request URI: http://localhost:8001/api/xxx/xxx]
    [HTTP request 6/6]
    [Prev request in frame: 97]
    [Response in frame: 107]
    File Data: 1127 bytes
MIME Multipart Media Encapsulation, Type: multipart/form-data, Boundary: "---------------------------20261806215495369501698203786"
    [Type: multipart/form-data]
    First boundary: -----------------------------20261806215495369501698203786\r\n
    Encapsulated multipart part: 
    Boundary: \r\n-----------------------------20261806215495369501698203786\r\n
    Encapsulated multipart part: 
    Boundary: \r\n-----------------------------20261806215495369501698203786\r\n
    Encapsulated multipart part: 
    Boundary: \r\n-----------------------------20261806215495369501698203786\r\n
    Encapsulated multipart part:  (image/png)
    Last boundary: \r\n-----------------------------20261806215495369501698203786--\r\n

Now i also have a Client written in C# using the HTTPClient, and i can't get the fileupload working. This is my code there right now:

        public async Task<string?> Addxxx(string name, string description, bool isPrivate, byte[] ImageData, CancellationToken token)
        {
            var url = $"{Scheme}://{Address}:{Port}/api/xxx/xxx";
            var Content = new MultipartFormDataContent();
            var ImageContent = new ByteArrayContent(ImageData);
            ImageContent.Headers.ContentType = MediaTypeHeaderValue.Parse("image/jpeg");
            Content.Add(ImageContent);
            var request = new HttpRequestMessage(HttpMethod.Post, url)
            {
                Content = new MultipartFormDataContent()
                {
                    {
                        new FormUrlEncodedContent(new List<KeyValuePair<string, string>>()
                        {
                            new KeyValuePair<string, string>("name" , name),
                            new KeyValuePair<string, string>("description" , description),
                            new KeyValuePair<string, string>("isprivate" , isPrivate.ToString()),
                        })
                    },
                    {Content }
                }
            };
            request.Headers.Add("Accept", "multipart/form-data");
            request.Headers.Add("User-Agent", UserAgent);
            request.Headers.Add("Cookie", Cookies);
            var response = await HttpClient.SendAsync(request, token);

            if (!response.IsSuccessStatusCode)
                return null;
            ...
        }

However, this always gets a bad request. This is the request generated:

Frame 16: 1523 bytes on wire (12184 bits), 1523 bytes captured (12184 bits) on interface \Device\NPF_Loopback, id 2
Null/Loopback
Internet Protocol Version 4, Src: 192.168.2.101, Dst: 192.168.2.101
Transmission Control Protocol, Src Port: 51121, Dst Port: 8001, Seq: 1394, Ack: 1020, Len: 1439
Hypertext Transfer Protocol
    POST /api/xxx/xxxHTTP/1.1\r\n
    Host: 192.168.2.101:8001\r\n
    Accept: multipart/form-data\r\n
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.116 Safari/537.36\r\n
     [truncated]Cookie: .AspNetCore.Cookies=CfDJ8M_ELI8knopFkQ6zEqeCkgx2TK33iweuVbSNkdR_HNs5nC6EEy6zFxnuwpIovyH9WenSzaVCDgW3VWk0BJJ7cDnfdnQ5W_o98sJvaVtiVmPk3JB8e5bySGdcoRN6NwIl27mqLMl89Kt5hnsrq5Jps9Q3kJ1XSk5kpMciycb9AijtG9N_tqqqhRbV9EvbQc0CUU
    Request-Id: |10c9eb89-46f0038fdc2e3b2d.\r\n
    Content-Type: multipart/form-data; boundary="4265ae7c-56da-4811-b646-fc1123794472"\r\n
    Content-Length: 576\r\n
    \r\n
    [Full request URI: http://192.168.2.101:8001/api/xxx/xxx]
    [HTTP request 3/3]
    [Prev request in frame: 10]
    [Response in frame: 20]
    File Data: 576 bytes
MIME Multipart Media Encapsulation, Type: multipart/form-data, Boundary: "4265ae7c-56da-4811-b646-fc1123794472"
    [Type: multipart/form-data]
    First boundary: --4265ae7c-56da-4811-b646-fc1123794472\r\n
    Encapsulated multipart part:  (application/x-www-form-urlencoded)
    Boundary: \r\n--4265ae7c-56da-4811-b646-fc1123794472\r\n
    Encapsulated multipart part:  (multipart/form-data)
    Last boundary: \r\n--4265ae7c-56da-4811-b646-fc1123794472--\r\n

I tried a lot and am a bit lost in how to Send the request from the Client... Can someone point me in the right direction on how to send the image together with the other POST data?