Dooku is the obvious choice, which might make one think that is the reason it isn't him.
One theory I read has Anakin erasing the file under the guidance of Palpatine/Sidious.
Another has Yoda doing it... I don't recall the reasoning behind this one.
The most recent is that Mace is the traitor. The idea being that whomever did it has to be a major player and not just one of the background Jedi. Lucas says that he wanted to keep the loyalties of Dooku secret until the end of the movie and still those loyalties could be questioned. What if mace was following Dooku's story on trying to create a better republic not knowing that he had any relationship with Sidious? There is no specific dialogue to quote that would help this theory along but just in case you need another reason to watch the movie, watch it again with this in mind.
When they meet at the beginning in Palpatine's office, they all get up to greet Amidala, except mace who sits there rubbing his chin pondering something. At first I thought that maybe mace had detected the dark side in Palpatine and was trying to organize those thoughts. With this theory, what if this is just part of a struggle that mace his having with himself over the right and wrong of what he has been doing with Dooku.
When Palpatine suggests that the Jedi protect Amidala, Yoda gives Palpatine that look, like he suspects something while mace is quick to agree.
When obi-wan expresses his concerns about Anakin to Yoda and mace, Yoda seems to agree with obi-wan until mace speaks up and reminds them of the prophecy.
Mace suggests that they reveal their weakness to the senate, which makes no sense, and Yoda is quick to let him know this.
Mace is strong with the force, but to sneak up on Dooku and Jango at the arena seems like quite a task, since Dooku is probably more powerful than mace. Also, Dooku does not seem surprised by mace's appearance.
After Jango goes after mace and mace takes Jango's head off, mace gives Dooku a look, possibly a look of betrayal?
Why does mace not go after Dooku while other Jedi do?
I'm not saying that I think this is what is really going on, but I think you'll find it interesting if you watch the movie again with all of this in mind. I think it would be a bad move for Lucas to have mace end up being a bad guy, but what if he had been struggling with what he was doing with Dooku was right or wrong and the arena battle is mace's revelation and puts him back on the "good" path. Let's also not forget that the fighting style of mace is borderline "dark side" style. Apparently on the strongest of Jedi can use this style w/o turning to the dark side. Perhaps mace's thoughts are in this gray area.
Let us not forget what Yoda says... "Do not assume anything, clear your mind must be if you are to discover the real villians behind this plot"