[HttpPost]
public async Task<IActionResult> XCXOCR(string text1, string text2, string text3, IFormFile fileInput)
{
    Result _Result = new() { Error = 0, Message = "获取编号成功！" };
    try
    {

        string _token = "103_vcn-JoX7KqJY1HvQ_nRdpLPZvzWSfuYbvUPWAFO2cEappJE27faSKndA7pSHfp4h1d7ptBYE-H_CIV_uoxn1xlPL-Yum-LWOUuAu3Q970wmbFc49i5woJI_WODMIRAdAAATCL";
        IFormFile _ReceiveFile = fileInput;
        var ext = System.IO.Path.GetExtension(_ReceiveFile.FileName).ToLowerInvariant();

        if (string.IsNullOrEmpty(ext) || !permittedExtensions.Contains(ext))
        {
            _Result.Error = 8;
            _Result.Message = "上传的文件格式不正确！";
            goto ExitProgram;
        }
        bool _isfileformat = false;
        _ReceiveFile.OpenReadStream().Seek(0, SeekOrigin.Begin);

        using (var reader = new BinaryReader(_ReceiveFile.OpenReadStream()))
        {
            var signatures = _fileSignature[ext];
            var headerBytes = reader.ReadBytes(signatures.Max(m => m.Length));

            _isfileformat = signatures.Any(signature =>
                 headerBytes.Take(signature.Length).SequenceEqual(signature));
        }
        if (!_isfileformat)
        {
            _Result.Error = 9;
            _Result.Message = "上传的文件签名不正确！";
            goto ExitProgram;
        }
        try
        {
            _ReceiveFile.OpenReadStream().Seek(0, SeekOrigin.Begin);
            using (var imgBmp = new System.Drawing.Bitmap(_ReceiveFile.OpenReadStream()))
            {
            }

        }
        catch
        {
            _Result.Error = 10;
            _Result.Message = "上传的并非图片文件！";
            goto ExitProgram;

        }

        string apiUrl = $"https://api.weixin.qq.com/cv/ocr/comm?access_token={_token}";
        using (var httpClient = new HttpClient())
        {

            // 1. 读取文件内容
            byte[] fileBytes;
            using (var ms = new MemoryStream())
            {
                await _ReceiveFile.CopyToAsync(ms);
                fileBytes = ms.ToArray();
            }

            string boundary = "------------------------" + DateTime.Now.Ticks.ToString("x");




            byte[] multipartBody = BuildMultipartBody(fileBytes, _ReceiveFile.FileName, _ReceiveFile.ContentType, boundary);

            // 3. 创建 ByteArrayContent 并设置 Content-Type 头
            var byteContent = new ByteArrayContent(multipartBody);

            // 正确方式：使用 MediaTypeHeaderValue 构造函数，分别传入媒体类型和参数
            var contentType = new MediaTypeHeaderValue("multipart/form-data");
            contentType.Parameters.Add(new NameValueHeaderValue("boundary", boundary));
            byteContent.Headers.ContentType = contentType;

            // 4. 发送请求
            HttpResponseMessage response = await httpClient.PostAsync(apiUrl, byteContent);
            string responseBody = await response.Content.ReadAsStringAsync();
            // 4. 处理响应
            if (response.IsSuccessStatusCode)
            {

                _Result = new() { Error = 0, Message = $"获取机器编号成功！", Data = responseBody };
            }
            else
            {

                _Result = new() { Error = 11, Message = $"调用失败，状态码: {response.StatusCode}", Data = responseBody };

            }

        }
            
    }
            
    
     catch (Exception ex)
    {
        _Result = new() { Error = -1, Message = "错误见信息！", Data =ex.ToString() };
    }
ExitProgram:
    //return JsonSerializer.Serialize(_Result);
    // return $"{{\"Error\":{_Result.Error},\"Message\":\"{_Result.Message}\",\"Data\":\"{_Result.Data}\"}}";
    // return Ok(_Result);
    // var response = new { Success = true, Data = "" };
    return Ok(_Result);
}

private byte[] BuildMultipartBody(byte[] fileData, string fileName, string contentType, string boundary)
{
    using var stream = new MemoryStream();
    var encoding = Encoding.UTF8;

    // 第一部分：文件部分头部
    string header = $"--{boundary}\r\n" +
                    $"Content-Disposition: form-data; name=\"img\"; filename=\"{fileName}\"\r\n" +
                    $"Content-Type: {contentType}\r\n\r\n";
    byte[] headerBytes = encoding.GetBytes(header);
    stream.Write(headerBytes, 0, headerBytes.Length);

    // 文件内容
    stream.Write(fileData, 0, fileData.Length);

    // 结束 boundary
    string footer = $"\r\n--{boundary}--\r\n";
    byte[] footerBytes = encoding.GetBytes(footer);
    stream.Write(footerBytes, 0, footerBytes.Length);

    return stream.ToArray();
}
试试这个代码

{"errcode":41005,"errmsg":"media data missing hint: [euKmIA0205w972] rid: 648cfc34-1302fb5b-5633b4eb"}