This plugin limits it's use to html files, but I don't see why it couldn't be changed to preview html code from all file types including unsaved files.

What preview would you want to see for other file types? 

Version [v1.3.1.0] attempts showing a preview of other file types as well; you can specify what command should be performed to preview that file. But in general, it doesn't make much sense to preview plain text when plain text is clearly visible in Notepad++ itself.

Unfortunately, 1.3 appears to have some fundamental stability issues.