In the multi-value select box users need to know how to write the attribute. If your system users know this information, it can be a good option, but if there's some ambiguity in which attributes are avaiable or how are they written, I think another approximation would be better.
If you feel that there are too many attributes to use checkboxes, I think you could use a more familiar pattern like the two lists selection: one list has all the attributes available (that can be filtered, searched, etc.) and another list contains selected attributes. Users can select one or more attributes from the first list and add them to the second one.