I usually create a "file filter" instead. Keep the extension as is (hi.html) and then use the try_files directive in nginx to make the server look for files with the same path using .html as an example. It has been a while since I used nginx but it looks something like this:

try_files $uri.html

Is it? Assigning text/html to extensionless file would be:

    <FilesMatch "^[^.]+$">
        ForceType text/html
    </FilesMatch>

Which I don't think is that bad, compared to RewriteRule's.