In server-directory\1.x\main\plugin-server-directory\pages\services\server-directory\common\files_3.3.xsl

<xsl:param name="header" select="ametys:serviceParameter('header')"/>

But the name of the param is not "header", it is "title"

However, the right name seems to be header because in attachments and explorer folder services this parameter is named header. See the linked issue.