Mengirim data berdasarkan session

Permisi, saya mau tanya. Bagaimana saya mau mengirim data ke session yang di tuju saja. Jadi saya ingin mengirim file untuk ke session yang di tuju saja. dan session tersebut di tampilkan melalui combobox ( bisa di lihat di code html ).

if (isset($_POST['simpan']))
{


    $user 			= $_SESSION['user'];
	$allowed_ext	= array('pdf');
	$file_name		= $_FILES['file_dokumen']['name'];
	$file_ext		= strtolower(end(explode('.', $file_name)));
	$file_size		= $_FILES['file_dokumen']['size'];
	$file_tmp		= $_FILES['file_dokumen']['tmp_name'];

	$namaFile		= mysqli_real_escape_string($link, $_POST['nama_file']);

	$randFile 		= uniqid().rand(0,999).$namaFile;

	if(in_array($file_ext, $allowed_ext) === true){
		if($file_size < 21044070){
		    $lokasi = 'upload_file/'.$randFile.'.'.$file_ext;
			move_uploaded_file($file_tmp, $lokasi);
			$in = mysqli_query($link,"INSERT INTO tm_file(nama_file,file_dokumen,id_user) VALUES('$namaFile', '$lokasi', '$user') where username = '$username'");
			if($in){
				echo "<script>alert('sukses');window.location.assign(\"page.php?page=file\")</script>";
			}else{
				echo "<script>alert('sukses');window.location.assign(\"page.php?page=file\")</script>";
			}
		}else{
			echo "<script>alert('ERROR: Besar ukuran file (file size) maksimal 1 Mb!');window.location.assign(\"page.php?page=file\")</script>";
		}
	}else{
		echo "<script>alert('ERROR: Ekstensi file tidak di izinkan!');window.location.assign(\"page.php?page=file\")</script>" ;
	}
}

?>

itu script untuk mengirim filenya. untuk htmlnya

 <div class="modal-body">



					                                <form role="form" method="post" enctype="multipart/form-data">

					                                	<div class="form-group"><label class="col-sm-3 control-label">Nama File</label>
					                                    	<input type="text" class="form-control" name="nama_file" placeholder="nama file" required="required"/>
					                                    </div>
					                                	<div class="form-group">
					                                		<label>Upload File PDF</label>
									                            <div class="fileinput fileinput-new" data-provides="fileinput">
									                                <span class="btn btn-default btn-file"><span class="fileinput-new">Select file</span><span class="fileinput-exists">Change</span>
									                                <input type="file" name="file_dokumen" ></span>
									                                <span class="fileinput-filename"></span>
									                                <a href="#" class="close fileinput-exists" data-dismiss="fileinput" style="float: none">×</a>
									                            </div>
					                                	</div>
					                                		<div class=\"form-group\">
															    <select name=\"username\">
															      <option value=\'belum memilih\' selected>- Pilih User -</option>
								                                		<?php
								                                		 $query = "select * from tm_user";
																	    	 $result = mysqli_query($link,$query);
																	    	 if(mysqli_num_rows($result)>0){
																	    	 	while ($row = mysqli_fetch_array($result)){
																	    	 	$b ="<option value=$row[username]>$row[username]</option>";
																	    	 	}
																	    	 }
																	    	 echo $b;
								                                		 ?>
					                                			</select>
														</div>


					                                    <div class="modal-footer">
				                                            <button type="button" class="btn btn-white" data-dismiss="modal">Batal</button>
				                                            <button type="submit" class="btn btn-primary" name="simpan">Simpan</button>
				                                        </div>
					                                </form>



                                        </div>

mohon di bantu , terima kasih