How to set the allowed media types in activity page using Buddypress and rtMedia WordPress plugin?

Copy the codes below to your functions.php

add_filter('rtmedia_plupload_files_filter', 'rt_custom_allowed_types', 10, 1);
function rt_custom_allowed_types( $types ){
	global $bp;
	if ( $bp->current_component == 'activity' && isset( $types[0] ) && isset( $types[0]['extensions'] ) ) {
		$types[0]['extensions'] = 'jpg,jpeg,png,gif';
	}
	return $types;
}

For more information please read this article https://rtmedia.io/docs/developers/allow-custom-media-types/