Apparently, there's very little information so far; the article starts by saying that Whedon is "said to be nearing a deal to write, direct and produce" such a film. Copied and pasted:
"No official word yet on any other details about the film, up to and including which version of Batgirl is likely to appear. Sources have told Entertainment Weekly that the film will take on the Barbara Gordon version of the character. More specifically, they claim the film will be based on the New 52 version of the character. Written by Gail Simone, this version of Babs had recently recovered from a spinal injury following her shooting by the Joker in The Killing Joke. During the series, she grapples with everything from Batwoman to the Joker to the Court of Owls, all while struggling to overcome her PTSD and return to the role she once played in the streets of Gotham."