It now correctly says "Do you want to open this link in Safari?"

Thanks a lot for the detailed description! It is certainly a very good idea to offer some predefined key sets and allow to people to create their own.

Should this option be enabled for all kinds of files or only HTML?

For example, php commands wouldn't be interpreted by UIWebView but people might still want to see it.

There is also another idea: "Upload and Download Files from and to Dropbox". Be sure to vote only for the feature variant you want to have implemented.