Do you use the File Viewer plugin at https://elgg.org/plugins/2648090? It should work on Elgg 2.3. It supports a number of file types already. For other file types I think you need to provide a view in views/default/file/specialcontent where the exact location within depends on the mimetype of the file. The view would deal with displaying the file. For some content you probably need to make use of additional libraries and other other code elsewhere to the plugin. But that would be a task for you to study all code of the plugin to understand how the different parts work together and how to add additional functionality.

Yes, I am using the File Viewer plugin at https://elgg.org/plugins/2648090, but it's not showing the content because it sends the download URL to the render engine using the function elgg_get_download_url($file).  This function is not returning a value.

I resolved the issue.  In my version upgrades, the 'filestore::dir_root' metastring string value did not get updated to the new filestore location.  Bullocks!