Loading the document into google docs first, and then formatting it, doesn't work when the RTF formatting is only created on the fly while the text is copied.
For example, when I'm writing code in Visual Studio, it is colored based on the context - comments are green, types are blue, etc. The file that is saved out is a
.txt, but when I copy a segment of text it is copied in RTF format, which I can then paste into other applications - e.g. MS Word.
As best I can tell, the answer is "you can't," at least not directly.
One workaround that does work is to paste into MS Word, then re-copy and paste from there to Google Docs. I'm sort of hoping that there's an extension that will make this easier, but I haven't found one yet.
Google tries to justify this by saying "they're different apps with different formats," but what they really mean is "when we coded our brand new app, long after the standards were nice and mature, we decided not to support the standard formats that everybody else has been using for years... but please, use our non-standard app for EVERYTHING, so you'll be locked into only using us in the future and we can mine everything you do for marketing data!"