<asp:FileUpload runat="server" ID="fileUpload_hovertree" ClientIDMode="Static" accept="image/png,image/jpeg,image/gif" />

HtPictureInfo h_info = new HtPictureInfo();
 h_info.HtSuffix = HoverTreeImageTool.GetGpjImageFileExtension(fileUpload_hovertree.PostedFile.ContentType);
 if (h_info.HtSuffix == "")
 {
 literal_tips.Text = "请选择jpg,png或者gif图片文件";
 return;
 }

namespace HoverTree.HoverTreeFrame.HtImage
{
 public class HoverTreeImageTool
 {
 /// <summary>
 /// 根据图片文件的mime内容类型获取文件的后缀名，如果不是gif,png或者jpg图片文件则返回空字符串
 /// http://hovertree.com/h/bjag/viv8qlpx.htm
 /// http://hovertree.com/texiao/h/contenttype/
 /// </summary>
 /// <param name="contentType"></param>
 /// <returns></returns>
 public static string GetGpjImageFileExtension(string contentType)
 {
 switch (contentType)
 {
 case "image/jpeg":
 return "jpg";
 case "image/pjpeg":
 return "jpg";
 case "image/gif":
 return "gif";
 case "image/png":
 return "png";
 case "image/x-png":
 return "png";
 default:
 return string.Empty;
 }
 }
 }
}

if (fileUpload_hovertree.PostedFile.ContentLength > 1048576)
 {
 literal_tips.Text = "选择的文件太大。";
 return;
 }