In ruby, stylistically you always try to write things in the positive. So:

```ruby
#bad
if !something
end

#good
unless something
end

Also, you use ! for logical not. not would be used with actions as would and/or vs. &&/|| (the latter for logical). Ruby Style Guide (RSG) discusses these nuances.

That being said, why not just:
```ruby
register_mime_type(mime_type, content_types) unless mime_type == :html
```