proposed fix: replacing implicit bool coercion w/ $.type(numberOfFiles)

*(apologies in advance, this is my very first pull request)*

If numberOfFiles === 0,  then the "maxNumberOfFiles" validation check
never fails, even if data.files.length exceeds the max (because 0 is
'falsy', the rest of the expression evaluates to false).   My proposed
fix is to  check the type of numberOfFiles.