Image Alt Text

function add_image_alt( $attachment_id ) {
	$image_mimes	 = array( 'image/jpeg', 'image/gif', 'image/png', 'image/bmp', 'image/tiff', 'image/x-icon' );
	$attachment_type = get_post_mime_type( $attachment_id );

	if( in_array( $attachment_type, $image_mimes ) ) {
		$attachment_title = get_the_title( $attachment_id );
		update_post_meta( $attachment_id, '_wp_attachment_image_alt', sanitize_text_field( $attachment_title ) );
	}
}
add_action( 'add_attachment', 'add_image_alt' );