This option is available until 6.1, but does not appear to be available in 6.2.However, I've checked the Release Notes and can find nothing about it being removed/deprecated.We will open an internal issue for this to be clarified and confirmed.
You can also look at the documentation:Add New Selection Category to Asset BrowserWhich references the github sample:https://github.com/Adobe-Marketing-Cloud/aem-authoring-extension-assetfinder-flickr
New information about extending Content Fragments has been published (and existing content updated). See:https://docs.adobe.com/docs/en/aem/6-2/develop/extending/content-fragments.htmlhttps://docs.adobe.com/docs/en/aem/6-2/develop/templates/content-fragment-templates.htmlhttps://docs.adobe.com/docs/...