Hello,
I had same issue and make this hook for it:
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 );