Unable to get ID3 data

I’m trying to retrieve the ID3 tags of an mp3 file and I’m using the following code for testing.

<?php
    $uploads = wp_upload_dir();
    $uploads_dir = ( $uploads['baseurl'] . $uploads['subdir'] );
    $file_path = $uploads_dir . "/song.mp3";
    $metadata = wp_read_audio_metadata( $file_path );

    echo "This song is played by " . $metadata['artist'];

    var_dump($metadata);
?>

I took the first 4 lines from an example in the codex, and $file_path does display the correct url when printed.

The echoed string doesn’t display the artist name and the var_dump returns a bool(false).

I pasted this code in header.php, but the result was the same.

if ( !function_exists( 'wp_read_audio_metadata' ) ) { 
    require_once ABSPATH . '/wp-admin/includes/media.php'; 
}

Strangely, the wp_read_audio_metadata function works perfectly fine when I use the ACF plugin to supply the file (even without including media.php), but directly providing an url doesn’t seem to work.

Am I missing something? Any help is appreciated!