The thing is, Manning didn't go into detail about any Gitmo files when chatting with Adrian Lamo, and, according to the Nation's Wikileaks pointsman Greg Mitchell, there were rumors that Wikileaks had secret Gitmo files before the alleged leaking from Manning took place. http://www.thenation.com/blog/160135/wikileaks-blog-guant%C3...
Remember, too, that Wikileaks released Gitmo's "Camp Delta Standard Operating Procedures" way back in 2007.
This leak could have come from anybody at any time. My feeling is that this was probably someone other than Manning, though I obviously don't have any real evidence to support that.