Manual:$wgNamespaceContentModels/en

Details
Associative array mapping namespace IDs to the name of the content model pages in that namespace should have by default (use the  constants). If no special content type is defined for a given namespace, pages in that namespace will use the  (except for the special case of JavaScript and CSS pages).

JSON
Suppose you have created a new custom namespace   and you want all new pages in this namespace to have JSON content and to be supported by the appropriate content handler. Add the following to your LocalSettings.php file.

$wgNamespaceContentModels[NS_JSON] = CONTENT_MODEL_JSON;

P.S. If you already created pages in this namespace, |see the talk page.