Sometimes we have a requirement to upload a custom HTML page or txt file on the Root folder of SharePoint sites. This could be used with Robots.txt or domain validation used with third party services like Google Apps or etc..
The best way to accomplish this task is to use SharePoint Designer.
Just open SharePoint Designer and login to your site.
In the Navigation Section, Under Site Objects, double click on ‘All Files’.
Now simply drag & drop the custom HTML page from your PC (Desktop) to the right pane of SharePoint Designer, under ‘All Files’ tab (as shown below).
Additionally you can make this HTML File as ‘Default Page’.
Just right click on Same page and choose ‘Set as Home Page’ from popup menu.
* Sorry for bad quality images.