"Download Zip" of results makes a zip file without an extension

When I click the "Download Zip" button on the results page of a successfully built circuit, I download a file called 'Unknown'. This file is an uncorrupted zip file missing the extension and containing the results directory of interest but OSX doesn't seem to be able to recognize the file without it explicitly given.  If I add the extension (renaming to 'Unknown.zip'), I can extract no problem. 