The 1.x version of the .NET driver allowed applications to use any value they wanted to for the file id of an uploaded GridFS File. The GridFS spec used to guide development of the 2.x version of the .NET driver GridFS API specified that the file id MUST be of type ObjectId and that the value would be generated by the driver itself.
It has since become evident that there are valid use cases for allowing the application to set its own value for the file id when uploading a GridFS file, including using other types of file id besides ObjectId.
The GridFS spec is being revised to allow applications to provide custom file id values when uploading GridFS files. The .NET driver will be updated accordingly.