I think you're going to have to put in a feature request for this with Now Support. The "mandatory attachment" is not a required field, but rather an "option" (attribute) of the catalog item itself...whereas the "required information section" (widget) on the portal is looking at fields on the form which are mandatory, but not yet populated with a value - and "attachment" is not a field, so it is not being caught by that widget.
In the mean time, you could simulate this in a "round about way" by creating a string variable on your catalog item called "attachment" which is hidden but mandatory (no way to fill it out, but will display as "mandatory" in the required information section), then write a client script which changes the variable's value to something (like "done") once an attachment is added...which would then satisfy the "mandatory field" aspect of your custom (fake) "attachment variable" - and it will no longer show in "required information".
