Don't check folder names against file whitelist

Previously, AttachmentUploader::Zipfile#filenames was returning the path
to both files and folders, which would cause problems when a folder
name contained a "." in it. This is because everything after the "." was
treated as a file extension and checked against the whitelist. For
example, given "some/folder/xml_schema_v2.4/", the file extension
validation would treat ".4/" as the file extension and thus the zipfile
would flunk validation. This enhances the code that grabs the filenames
from the zip so that it's only the filenames that are returned, and not
any folders.