public class UpoladService : IUpoladService
{
    private readonly static string[] _imageExtensions = new string[] { ".jpg", ".png", ".gif", ".jpeg", ".bmp" };
    private AppSettings _appSettings;

    public UpoladService(IOptions<AppSettings> appSettings)
    {
        _appSettings = appSettings.Value;
    }

    public async Task<SubmitResult> UploadStream(Stream stream, string fileName, AppType appType)
    {
        if (stream == null)
        {
            return SubmitResult.Fail("图片为null");
        }
        if (string.IsNullOrWhiteSpace(fileName))
        {
            return SubmitResult.Fail("图片名称为空");
        }
        try
        {
            var extensionName = fileName.Substring(fileName.LastIndexOf("."));
            if (!_imageExtensions.Contains(extensionName.ToLower()))
            {
                return SubmitResult.Fail("图片格式有误");
            }
            var generateFileName = $"{DateTime.Now.ToString("yyyyMMddHHmmssfff-")}{Guid.NewGuid().GetHashCode().ToString().Replace("-", string.Empty)}{extensionName}";
            var saveKey = $"wl/{appType.ToString().ToLower()}/{generateFileName}";

            // 生成(上传)凭证时需要使用此Mac
            // 这个示例单独使用了一个Settings类，其中包含AccessKey和SecretKey
            // 实际应用中，请自行设置您的AccessKey和SecretKey
            Mac mac = new Mac(_appSettings.AccessKey, _appSettings.SecretKey);
            // 上传策略，参见 
            // https://developer.qiniu.com/kodo/manual/put-policy
            PutPolicy putPolicy = new PutPolicy();
            // 如果需要设置为"覆盖"上传(如果云端已有同名文件则覆盖)，请使用 SCOPE = "BUCKET:KEY"
            // putPolicy.Scope = bucket + ":" + saveKey;
            putPolicy.Scope = _appSettings.Bucket;
            // 上传策略有效期(对应于生成的凭证的有效期)   
            putPolicy.SetExpires(3600);
            // 上传到云端多少天后自动删除该文件，如果不设置（即保持默认默认）则不删除
            //putPolicy.DeleteAfterDays = 1;

            // 生成上传凭证，参见
            // https://developer.qiniu.com/kodo/manual/upload-token            
            string jstr = putPolicy.ToJsonString();
            string token = Auth.CreateUploadToken(mac, jstr);

            FormUploader fu = new FormUploader();
            var result = await fu.UploadStreamAsync(stream, saveKey, token);
            if (result.Code == 200)
            {
                return SubmitResult.Success($"{_appSettings.Domain}/{saveKey}");
            }
            return SubmitResult.Fail("上传失败");
        }
        catch (Exception ex)
        {
            return SubmitResult.Fail($"上传失败：{ex.Message}");
        }
    }
}

[EnableCors("default")]
[Route("upload")]
public class UploadController : WebApiController
{
    private IUpoladService _upoladService;

    public UploadController(IUpoladService upoladService)
    {
        _upoladService = upoladService;
    }

    [Route("{appType}/stream")]
    [HttpPost]
    public async Task<IActionResult> UploadByStream(AppType appType)
    {
        if (Request.Form.Files.Count == 0)
        {
            return Fail("图片不存在");
        }
        var file = Request.Form.Files[0];
        var result = await _upoladService.UploadStream(file.OpenReadStream(), file.FileName, appType);
        if (result.IsSuccess)
        {
            return Ok(new { pic_url = result.Data, is_success = true });
        }
        return Fail(result.Message);
    }

    [Route("{appType}/base64")]
    [HttpPost]
    public async Task<IActionResult> UploadByBase64([FromBody]JObject data, AppType appType)
    {
        var imageBase64 = data["image_base64"].ToObject<string>();
        var fileName = data["file_name"].ToObject<string>();
        if (!string.IsNullOrWhiteSpace(imageBase64))
        {
            var reg = new Regex("data:image/(.*);base64,");
            imageBase64 = reg.Replace(imageBase64, "");
            byte[] imageByte = Convert.FromBase64String(imageBase64);
            var stream = new MemoryStream(imageByte);

            var result = await _upoladService.UploadStream(stream, fileName, appType);
            if (result.IsSuccess)
            {
                return Ok(new { pic_url = result.Data, is_success = true });
            }
            return Fail(result.Message);
        }
        return Json(new { success = false, result = "请选择文件！" });
    }
}

<form method="post" enctype="multipart/form-data">
    <input type="file" id="files"
            name="files" multiple />
    <input type="button"
            id="uploadStream"
            value="Upload Selected Files" />

    <input type="button"
            id="uploadBase64"
            value="Upload Selected Files" />
</form>

<script>
$(document).ready(function () {
    $("#upload").click(function (evt) {
        var fileUpload = $("#files").get(0);
        var files = fileUpload.files;
        var data = new FormData();
        for (var i = 0; i < files.length; i++) {
            data.append(files[i].name, files[i]);
        }
        $.ajax({
            type: "POST",
            url: "http://localhost:5002/upload/avatar/stream",
            processData: false,
            contentType: false,
            data: data,
            success: function (data) {
                if (data.is_success) {
                    console.log(data.pic);
                }
                else {
                    console.log(data.msg);
                }
                console.log(data);
            },
            error: function () {
                console.log("There was error uploading files!");
            }
        });
    });

    $("#uploadBase64").click(function (evt) {
        var fileUpload = $("#files").get(0);
        var files = fileUpload.files;
        var data = {};

        //生成base64
        var reader = new FileReader();
        reader.readAsDataURL(files[0]);
        reader.onload = function () {
            data.image_base64 = reader.result;
            data.file_name = files[0].name;

            $.ajax({
                type: "POST",
                url: "http://localhost:5002/upload/avatar/base64",
                contentType: 'application/json; charset=utf-8',
                data: JSON.stringify(data),
                dataType: 'json',
                success: function (data) {
                    if (data.is_success) {
                        console.log(data.pic);
                    }
                    else {
                        console.log(data.msg);
                    }
                    console.log(data);
                },
                error: function () {
                    console.log("There was error uploading files!");
                }
            });
            return reader.result;
        };
        reader.onerror = function (error) {
            console.log('Error: ', error);
        };
    });
});
</script>

本文转自田园里的蟋蟀博客园博客，原文链接：http://www.cnblogs.com/xishuai/p/asp-net-core-upload-image-with-qiniu.html，如需转载请自行联系原作者