如何使用Mongo-go-driver查找与gridfs一起存储的一个文件并更新内容？

My aim is assemble chunked file get from HTTP and stored in GridFS

Here what I've done so far:

var gridfs
// How to implement this code
if gridfs.fs.exists("object_id"){
    // How to implement this code
    gridfs.fs.upload("the other part content")
}else{
    // code here i implemented 
    gridfs.fs.createfs("file_name", "first part content")
}