fix for StaticAssets when a file has an unrecognized mime type

If a file in a StaticAssets directory has an invalid mime type, it will short-circuit the forEachSeries: next() will not be called. So the rest of the files won't be added to the static assets. 

Eg. for this directory structure:
/static
   test.css
   zzz.file

zzz.file will never get next() called, and the next files won't get added to the static assets.