FileHandler in SharePoint Online for External User Collaboration

I am trying to create the file handler for extension say ".abc" in SharePoint Online. This works fine when the file is opened by any user of the azure active directory. But in case of file sharing from SharePoint Online to external user, the file handler is not called. I have already tried the following Created the file handler as Enterprise app and allowed it to be accessible across tenants. How can i configure the file handler to be called for External users in SharePoint online?