Set Content-Disposition header to attachment only on files in a certain directory?

I've got this this rule in my htaccess file to force linked files to download rather than open in the browser:

<FilesMatch "\.(gif|jpe?g|png)$">
  ForceType application/octet-stream
  Header set Content-Disposition attachment
</FilesMatch>

Is there a way to alter the RegExp so it only applies to files in a certain directory?

Thanks