Ecco:

$idfile=$_GET['dintPkId'];
$result = $myconn->query('SELECT intPkId,intBidId,strName,binFile FROM tblDocuments WHERE intPkId = "'.$idfile.'"') or die("Errore...query" . mysqli_error($result));
$tmp = mysqli_fetch_array($result);
$filename=$tmp['strName'];

//Gather relevent info about file
$len = $tmp['binFile'];
$name = $tmp['strName'];
$ext = strtolower(substr(strrchr($name,"."),1));

//This will set the appropriate Content-Type
switch( $ext ) {
case "pdf": $ctype="application/pdf"; break;
case "exe": $ctype="application/octet-stream"; break;
case "zip": $ctype="application/zip"; break;
case "doc": $ctype="application/msword"; break;
case "xls": $ctype="application/vnd.ms-excel"; break;
case "ppt": $ctype="application/vnd.ms-powerpoint"; break;
case "gif": $ctype="image/gif"; break;
case "png": $ctype="image/png"; break;
case "jpeg":
case "jpg": $ctype="image/jpg"; break;
case "mp3": $ctype="audio/mpeg"; break;
case "wav": $ctype="audio/x-wav"; break;
case "mpeg":
case "mpg":
case "mpe": $ctype="video/mpeg"; break;
case "mov": $ctype="video/quicktime"; break;
case "avi": $ctype="video/x-msvideo"; break;

//Following are extensions that shouldn't be downloaded
case "php":
case "htm":
case "html":
case "txt": die("Cannot be used for ". $ext ." files!");
break;

default: $ctype="application/force-download";
}

//Begin writing headers
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate,
post-check=0, pre-check=0");
header("Cache-Control: public");
header("Content-Description: File Transfer");

//Use the switch-generated Content-Type
header("Content-Type: $ctype");

//Force the download
$header="Content-Disposition:attachment;filename=".$name;
header($header );
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".$len);
@readfile($file);
exit;


//Function call
$file =$_REQUEST['file'];
download_file($file);