External files can be uploaded for use in notebooks by doing the following:
In your notebook, go to the "File > Open" menu. You should see an "Upload" file button at the top. Use that to save your external file in your workspace. Instructions on how to save files in a bucket to permanently save your file can be found in the following support article, How do I use Google buckets to save my files in notebooks?
Containers can also be used to upload files, but they need to be on the Google Cloud Registry.