add file size to file stat

The preview images feature needs to show the size of the image. There is no easy way to get the files size of an image from JS, therefore it seemed appropriate to add this functionality to the existing API GetFileModificationTime

It seems the name of that API is obsolete, I'd be happy to update the name of the call at this time too. I'm just wondering why this wasn't done before when the functionality to determine wether a path is a file or folder was added