Thanks for your response.
Currently, we are unable to provide you any particular date. We release a new version of the GroupDocs.Viewer for .NET in the second half of each month and it is planned to include this feature in any of the upcoming releases. If everything goes according to the plan, hopefully, it will take a month or two to provide the support of this feature.
Furthermore, there are no particular file types that are to be rendered as image to get the best results. In the above post, the complex document meant the one that may contain various graphic elements such as complex charts and graphs, different designs, different formatting styles etc. However, GroupDocs.Viewer promises the accuracy in both, HTML based rendering and image based rendering.
In case you would have any confusion, please do let us know.