ASP.NET在上传文件时对文件类型的高级判断的代码

using System; 
using System.Data; 
using System.Configuration; 
using System.Collections; 
using System.Web; 
using System.Web.Security; 
using System.Web.UI; 
using System.Web.UI.WebControls; 
using System.Web.UI.WebControls.WebParts; 
using System.Web.UI.HtmlControls; 

public partial class _Default : System.Web.UI.Page 
{ 
protected void Page_Load(object sender, EventArgs e) 
{ 

} 
protected void bt_upload_Click(object sender, EventArgs e) 
{ 
try 
{ 
if (FileUpload1.PostedFile.FileName == "") 
{ 
this.lb_info.Text = "请选择文件！"; 
} 
else 
{ 
string filepath = FileUpload1.PostedFile.FileName; 
if (IsAllowedExtension(FileUpload1) == true) 
{ 
string filename = filepath.Substring(filepath.LastIndexOf("\\") + 1); 
string serverpath = Server.MapPath("images/") + filename; 
FileUpload1.PostedFile.SaveAs(serverpath); 
this.lb_info.Text = "上传成功！"; 
} 
else 
{ 
this.lb_info.Text = "请上传图片"; 
} 
} 
} 
catch (Exception error) 
{ 
this.lb_info.Text = "上传发生错误！原因：" + error.ToString(); 
} 
} 
public static bool IsAllowedExtension(FileUpload hifile) 
{ 
System.IO.FileStream fs = new System.IO.FileStream(hifile.PostedFile.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read); 
System.IO.BinaryReader r = new System.IO.BinaryReader(fs); 
string fileclass = ""; 
byte buffer; 
try 
{ 
buffer = r.ReadByte(); 
fileclass = buffer.ToString(); 
buffer = r.ReadByte(); 
fileclass += buffer.ToString(); 

} 
catch 
{ 

} 
r.Close(); 
fs.Close(); 
if (fileclass == "255216" || fileclass == "7173")//说明255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar 
{ 
return true; 
} 
else 
{ 
return false; 
} 
} 
}