how to upload the particular files from fileupload control in asp.net

Try this - It works correctly

if (fileUpload.HasFile)
{
System.IO.FileInfo sr = new System.IO.FileInfo(Server.MapPath(fileUpload1.FileName));
     if (sr.Extension == ".txt")
     {
         //Your Code to Save File
     }  
     else
     {
         lblStatus.Text="Select txt File";
     }
}