Get metadata of all files in the media

I need to retrieve metadata of files that are not images such as word, excel or pdf documents.

I tried using get_post_meta( $post_id ); and wp_get_attachment_metadata( $attachment_id );

I'm only getting the following information when using get_post_meta( $post_id );:

Array
(
    [_wp_attached_file] => Array
        (
            [0] => 2015/03/test.xlsx
        )

    [_edit_lock] => Array
        (
            [0] => 1425404399:741
        )
)

Nothing for wp_get_attachment_metadata( $attachment_id );.