NullPointerException when parsing skin files

On some systems (container specific) getMimeType return null for javascript files, causing a NPE in SkinAction#isJavascriptMimeType

A simple solution, which solves only the NPE, but not the real problem, is to use "javascript".equals(mimetype) instead of mimetype.equals("javascript").

A better solution would be to see why is null returned instead of the proper mimetype.