Search for Field is blank

[Trevor Watson]

I was wondering if there is a way to search for a field being blank.
We keep track of file extensions in our software and would like to do a 
search for files that don't have an extension.

We could do a search for all documents (we keep a "Generic all field" 
with "SelectAll" as the only Term) and then exclude * from the 
fileExtension field, but that could result in a ridiculous number of 
extensions that have to be run as "excluded".

Is there a way to search for field = blank?

Thanks in advance.

Trevor