This placeholder can't be used for custom file scanning. You need to specify the exact path and file name to be scanned. The only placeholders available are %windir%, %programfiles% and %programfiles(x86)%.
Recently we have posted a script
here which you could deploy using the deployment feature of Lansweeper. It locally scans computers for certain file types and writes the result into a TXT file. You might consider giving that a try.