How to get “Attach Article” of “Insert Article into Email” from Knowledge search results

I found how to do it.

As @scyforce said I need to use the key prefix.

Link on Salesforce Community

Code from that page:

    // Create DescribeMap
    private void createDescribeMap() {
        Map<String, Schema.SObjectType> gd = Schema.getGlobalDescribe();
        Set<String> keySet = gd.keySet();
        for (String key : keySet) {
            Schema.SObjectType objectType = gd.get(key);
            if (key.endsWith('ka') || key.endsWith('kb')) {
                this.describeMap.put(objectType.getDescribe().getKeyPrefix(), objectType.getDescribe().getLabel());
            }
        }
    }

    //Util method to get Article Type from article Id.
    public String getArticleType(String articleId) {
        String articlePrefix = articleId.substring(0,3);
        Set<String> keySet = describeMap.keySet();
        String articleType = null;
        for(String key: keySet) {
            if(articlePrefix.equalsIgnoreCase(key)) {
                articleType = describeMap.get(key);
                return articleType;
            }
        }
        return articleType;
    }

For most internal pages (behind authentication) within a community, if you attempt to hit the page unauthenticated, you are prompted to authenticate and then redirected to the page your were attempting to access. For some reason however, Knowledge does not work that way. If you attempt to access a Knowledge article that requires authentication and that is not exposed to the Public Knowledge Base, you receive a 404 and not a login prompt. If you wanted to create an IdeaExchange idea around this, I think it's a good one.

Therefore the only way to accomplish this is with your own creative workaround. You'll want to create a Visualforce page that handles the unauthenticated redirect, call it KnowledgeRedirect or something. This page will need to be exposed via the communities Sites settings to unauthenticated users (via Public Access Settings for the VF page). The page would accept a querystring parameter that is either the id or name of your knowledge article (some unique identifier). On hitting the page with that querystring param, you would first check to see if the user is authenticated. This should suffice in apex:

if (UserInfo.getUserType()=='Guest') { 
    //redirect to the login page with the querystring param on the startUrl or retUrl, I can't recall, one of those
} else {
    //redirect to the knowledge article
}