Quickly note "new" FilePath option in dev help

This feature seems to have popped up some time. Only a quick note because
this help tab is not prettified yet anyway.

resources/doc/help_dev.xml

@@ -528,6 +528,7 @@ their own simulation software. It showcases all panels and lists their options.
 				<li>When the optional <code>mode="input/output"</code> attribute is given, a few more checks are enabled about the file (e. g. an input file should exist).</li>
 				<li>Remember that these are warnings only, and the INI file will save normally.</li>
 				<li>You can supply <code><option extension="File type (*.ext)/>"</code> child nodes to filter for these file extensions.</li>
+				<li>A single extension can also be supplied via the "extension" attribute directly.</li>
 				&lt;/ul&gt;&lt;br&gt;</help>
 		</parameter>
 		<frame caption="File input">