GUI-800 added warning for filereader support if missing

added warning for filereader support if missing


https://eucalyptus.atlassian.net/browse/GUI-800
@dkavanagh: The content in the modal seems a bit misleading.  If the browser lacks support for the FileReader API, file uploads will still work.  We should be more specific in the messaging here.  Perhaps ask @JMoLo for input?