File extension fallback

If the mime-type is not specified in 

````
extractors = {
    '.zip': extractZip,
    '.tar': extractTar,
    '.tar.gz': extractTarGz,
    '.tgz': extractTarGz,
    '.gz': extractGz,
    'application/zip': extractZip,
    'application/x-tar': extractTar,
    'application/x-tgz': extractTarGz,
    'application/x-gzip': extractGz
};
````

then if falls back to checking if the source ends with any of the extensions in the extractors. This fixes cases where web servers may be configured to respond with `Content-type: application/octet-stream` for any of the common archive file extensions.