On WordPress singular attachment pages, send a custom Data Layer event to Google Tag Manager and then redirect to the raw file.

wordpress-track-and-redirect-media.php

<?php

add_action( 'wp_footer', function() {

	if ( !is_attachment() ) return;

	$file_url = wp_get_attachment_url( get_the_ID() );

	?><script>

		document.addEventListener('DOMContentLoaded', function(){

			var dataLayer = window.dataLayer;
			var fileURL   = '<?=esc_attr($file_url)?>';

			if ( typeof dataLayer != 'object' ) {
				window.location = fileURL;
				return;
			}

			dataLayer.push({
				event: 'wp_media_download',
				url:   fileURL,
				eventCallback: function( id ) {
					window.location = fileURL;
				}
			});

		});

	</script><?php

});