Using a keyword that contains a space in a Spotlight search?

I'd like to run a Spotlight search to find all email messages that contain a specific attachment:

attachment names:screenshot

I tried the keyword "attachment names" in the Spotlight search window and in Finder's search field, but it returns no results. I suspect that this may be due to the space in the keyword "attachment names".

When I define the search criteria in Finder and pick Attachment names from the menu, it does return the correct results (see screenshot #2, I cut out the actual results).

Note: I tried leaving out the space, but that did not work. However, for another keyword, "Color space", using "colorspace" does work...

Note 2: Attachments are only one example, I'm looking for a solution that works for any keyword that contains of more than one word.