一、 安全性比較低，把文本文件1.txt改成1.jpg照樣可以上傳，但其實現(xiàn)方法容易理解，實現(xiàn)也簡單，所以網(wǎng)上很多還是采取這種方法。

Boolean fileOk = false; 
          string path = Server.MapPath("~/images/"); 
          //判斷是否已經(jīng)選取文件 
          if (FileUpload1.HasFile) 
          { 
              //取得文件的擴(kuò)展名,并轉(zhuǎn)換成小寫 
              string fileExtension = System.IO.Path.GetExtension(FileUpload1.FileName).ToLower(); 
              //限定只能上傳jpg和gif圖片 
              string[] allowExtension = { ".jpg", ".gif" }; 
              //對上傳的文件的類型進(jìn)行一個個匹對 
              int j = 0; 
              for (int i = 0; i < allowExtension.Length; i++) 
              { 
                  if (fileExtension == allowExtension[i]) 
                  { 
                      fileOk = true; 
                      return; 
                  } 
                  else 
                  { 
                      j++; 
                  } 
              } 
              if (j > 0) 
              { 
                  Response.Write("<script>alert('文件格式不正確');</script>"); 
                  return; 
              } 
          } 
          else 
          { 
              Response.Write("<script>alert('你還沒有選擇文件');</script>"); 
              return; 
          } 
          //如果擴(kuò)展名符合條件，則上傳 
          if (fileOk) 
          { 
              FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName); 
              Response.Write("<script>alert('上傳成功');</script>"); 
          } 

二、不檢測文件后綴而是檢測文件MIME內(nèi)容類型。

Boolean fileOk = false; 
           string path = Server.MapPath("~/images/"); 
           //判斷是否已經(jīng)選取文件 
           if (FileUpload1.HasFile) 
           { 
               //取得文件MIME內(nèi)容類型 
               string type = this.FileUpload1.PostedFile.ContentType.ToLower(); 
               if (type.Contains("image"))    //圖片的MIME類型為"image/xxx"，這里只判斷是否圖片。 
               { 
                   fileOk = true; 
  
               } 
               else 
               { 
                   Response.Write("<script>alert('格式不正確')</script>"); 
               } 
           } 
           else 
           { 
               Response.Write("<script>alert('你還沒有選擇文件');</script>"); 
           } 
           //如果擴(kuò)展名符合條件，則上傳 
           if (fileOk) 
           { 
               FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName); 
               Response.Write("<script>alert('上傳成功');</script>"); 
           } 

三、可以實現(xiàn)真正意義上的文件類型判斷

try 
            { 
                //判斷是否已經(jīng)選取文件 
                if (FileUpload1.HasFile) 
                { 
                    if (IsAllowedExtension(FileUpload1)) 
                    { 
                        string path = Server.MapPath("~/images/"); 
                        FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName); 
                        Response.Write("<script>alert('上傳成功');</script>"); 
                    } 
                    else 
                    { 
                        Response.Write("<script>alert('您只能上傳jpg或者gif圖片');</script>"); 
                    } 
  
                } 
                else 
                { 
                    Response.Write("<script>alert('你還沒有選擇文件');</script>"); 
                } 
            } 
            catch (Exception error) 
            { 
                Response.Write(error.ToString()); 
            } 
            #endregion 
        } 
//真正判斷文件類型的關(guān)鍵函數(shù) 
        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 = ""; 
            //這里的位長要具體判斷. 
            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")//說明255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar 
            { 
                return true; 
            } 
            else 
            { 
                return false; 
            } 
  
        } 

原文鏈接：http://www.admin10000.com/document/1862.html