Hi @Zolotoy
I don't know much about C#, you can check out this documentation for SSRS exporting to HTML: Rendering to HTML (Report Builder and SSRS).
Regarding the HTML file extension, I found this link for you: https://www.scaler.com/topics/what-is-the-extension-of-html-file/.
If my answer is helpful to you, click "accept answer".
Best regards,
Aniya