<extensions>.ext1, .ext2, .ext</extensions>
It specifies a list of extensions that ZEST shall handle. If the extension of a request's URI does match any of them, it will be ignored (and handled as if ZEST is not installed).
By default, it is empty which means ZEST handles only URI without any extensions.
For example, the following statement indicates ZEST shall handle no extension,
<extensions>, .do, .action</extensions>
Notice that, to have better performance, ZEST matches the extension first before searching the action definitions. If you'd like to have an action definition matches a URI with an extension, make sure the extension of the URI is also specified here.