Source

File: wp-includes/post.php

function wp_attachment_is( $type, $post = null ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return false;
	}

	$file = get_attached_file( $post->ID );
	if ( ! $file ) {
		return false;
	}

	if ( 0 === strpos( $post->post_mime_type, $type . '/' ) ) {
		return true;
	}

	$check = wp_check_filetype( $file );
	if ( empty( $check['ext'] ) ) {
		return false;
	}

	$ext = $check['ext'];

	if ( 'import' !== $post->post_mime_type ) {
		return $type === $ext;
	}

	switch ( $type ) {
		case 'image':
			$image_exts = array( 'jpg', 'jpeg', 'jpe', 'gif', 'png' );
			return in_array( $ext, $image_exts, true );

		case 'audio':
			return in_array( $ext, wp_get_audio_extensions(), true );

		case 'video':
			return in_array( $ext, wp_get_video_extensions(), true );

		default:
			return $type === $ext;
	}
}