File not found error when using GridFSBucket openDownloadStream
Another possible explanation for this, if you're already calling openDownloadStream and still experiencing the FileNotFound error, and you are 100% the id is correct, is that you didn't pass an ObjectId type.
In my case, I was passing an id string instead of an id as an ObjectId.
bucket.openDownloadStream(mongoose.Types.ObjectId(id));vs
bucket.openDownloadStream(id);