ACF: Add file preview to admin panel

By adding this to your functions.php file you will be able to make a document preview of your uploaded file to the ACF File Upload field:

function acf_change_icon_on_files ( $icon, $mime, $attachment_id ){ // Display thumbnail instead of document.png
		
		if ( strpos( $_SERVER[ 'REQUEST_URI' ], '/wp-admin/upload.php' ) === false && $mime === 'application/pdf' ){
			$get_image = wp_get_attachment_image_src ( $attachment_id, 'thumbnail' );
			if ( $get_image ) {
				$icon = $get_image[0];
			} 
		}
		return $icon;
	}
	
	add_filter( 'wp_mime_type_icon', 'acf_change_icon_on_files', 10, 3 );