Storing & adding array in Laravel Session [duplicate]

I want to add the array value in Session. But it overwrite instead of adding new value.

This is the process of the function.

 public function example(){
    $fileName = $file . "_demo_a.htm";
    $demo["filename"][] = $fileName;
    Session::put($demo);
}

For example , when I run the functino for $file = a , the value is added in Session.But for the next time like $file = b , the value is overwritten.

The result what I want is like this ,

array (
  'filename' => 
  array (
    0 => 'a_demo.htm',
    1 => 'b_demo.htm'
  ),
)