Dear Asp Expert
              I tried uploading file with these lines of code
 .............
string srcfile=file1.PostedFile.FileName;
string desfile=Server.MapPath(@"./picture/"+txtTarget.Text);
file1.PostedFile.SaveAs(desfile);
................
         It worked well with all picture , text,pdf file but it didn't work with the .avi file .With the error message show that the server may encounter some difficulties.
          I tried the other way like this
..................
          string srcfile=@"d:/myvidio.avi";
          string descfile=Server.mapPath(@"./picture/"+txtTarget.Text);
           WebClient wc=new WebClient();
          wc.UpLoadFile(desfile,"POST",srcfile);
          wc.dispose();
.....................
           The file could be uploaded but could not open with windows media player .The error message show that it doesn't know the format of the file.
            What should I do next.Please tell me. thanks

What is the value of txtTarget.Text when you save the file?
The browser will understand many image formats automatically by the binary contents of the file. I'm not sure about PDF (unless the extension is specified).

After the file is uploaded is the file extension of the uploaded video "avi"? If so, then the client should be able to handle it whether the browser can play/show it or it can hand off the downloaded file to the appropriate helper application (media player). If not, then that's probably the problem.

-Peter