Mongoc_gridfs_file_get_id() is not same with _id in fs.files

Working with Data Drivers
1

Hello,
I want to add a uplodDate in fs.chunks collection, so I need get the files_id when I upload a file.But I found that mongoc_gridfs_file_get_id() returned is not same with _id in fs.files. How can I get the _id ?
Thanks