bp_attachments_get_allowed_types( string $type = 'avatar' )

function bp_attachments_get_allowed_types( $type = 'avatar' ) {
	// Defaults to BuddyPress supported image extensions.
	$exts = array( 'jpeg', 'gif', 'png' );

	/**
	 * It's not a BuddyPress feature, get the allowed extensions
	 * matching the $type requested.
	 */
	if ( 'avatar' !== $type && 'cover_image' !== $type ) {
		// Reset the default exts.
		$exts = array();

		switch ( $type ) {
			case 'video' :
				$exts = wp_get_video_extensions();
			break;

			case 'audio' :
				$exts = wp_get_video_extensions();
			break;

			default:
				$allowed_mimes = get_allowed_mime_types();

				/**
				 * Search for allowed mimes matching the type.
				 *
				 * Eg: using 'application/vnd.oasis' as the $type
				 * parameter will get all OpenOffice extensions supported
				 * by WordPress and allowed for the current user.
				 */
				if ( '' !== $type ) {
					$allowed_mimes = preg_grep( '/' . addcslashes( $type, '/.+-' ) . '/', $allowed_mimes );
				}

				$allowed_types = array_keys( $allowed_mimes );

				// Loop to explode keys using '|'.
				foreach ( $allowed_types as $allowed_type ) {
					$t = explode( '|', $allowed_type );
					$exts = array_merge( $exts, (array) $t );
				}
			break;
		}
	}

	/**
	 * Filter here to edit the allowed extensions by attachment type.
	 *
	 * @since BuddyPress 2.4.0
	 *
	 * @param array  $exts List of allowed extensions.
	 * @param string $type The requested file type.
	 */
	return apply_filters( 'bp_attachments_get_allowed_types', $exts, $type );
}