Add a new global config setting to aigaion:

* extend configeditforms.php by
    = making a form element for the new setting, possibly in a new section of the form
    = adding a sensible explanation text, and a sensible default value
    = (see other config settings for inspiration)
* extend pageactionconfig.php by
    = catching the value set in the editform above from the $_REQUEST (see other parameters...)
    = passing that new value on to the call to setConfig(..,..,..,..)
* extend 'configactions.php' by 
    = adding another parameter to setConfig(..,..,..,..)
    = if necessary, doing some processing on the value passed to this method (see e.g. $ALLOWED_ATTACHMENT_EXTENSIONS)
    = set the value of the new setting

Note, that in principle there is no new checkschema function needed...