The current button to add a filter to a certain type of email doesn't look for existing filters. Hence, it is not possible to add a new condition to an existing filter.
The user has to manually go to the settings, look for the filter and edit it.
This is specially painful when the number of filters is actually limited, and it increases clutter since the user is tempted to create redundant filters with just new conditions, and indexed naming strategies (Filter01, Filter01-Copy or whatever)...

Thanks.
Other than that, you're doing a great job, keep it going :D