ASP.NET - 上传图片方法（单张）

/// <summary>
/// 上传图片
/// </summary>
/// <param name="fileupload">上传的控件</param>
/// <param name="folder">要存储的文件夹（要在服务器已经存在）</param>
/// <param name="imageName">图片名称</param>
/// <returns></returns>
public string UploadImage(FileUpload fileupload, string folder, string imageName)
{
    Boolean fileOK = false;
    String path = Server.MapPath("~/" + folder + "/");
    if (fileupload.HasFile)
    {
        String fileExtension =
            System.IO.Path.GetExtension(fileupload.FileName).ToLower();
        String[] allowedExtensions = { ".gif", ".png", ".jpeg", ".jpg" };
        for (int i = 0; i < allowedExtensions.Length; i++)
        {
            if (fileExtension == allowedExtensions[i])
            {
                fileOK = true;
            }
        }
    }
    else
    {
        return "请选择要上传的文件！";
    }

    if (fileOK)
    {
        try
        {
            string ImagesPath = path + imageName + Path.GetExtension(fileupload.FileName);
            fileupload.PostedFile.SaveAs(ImagesPath);
            return "上传成功！";
        }
        catch
        {
            return "上传失败！";
        }
    }
    else
    {
        return "只能上传 .gif, .png, .jpeg, .jpg 等类型图片！";
    }
}