Fix FilesExt should fallback when Presigned URL is not available

[yuanjieding-db]

What changes are proposed in this pull request?

Provide the readers and reviewers with the information they need to understand
this PR in a comprehensive manner.

Specifically, try to answer the two following questions:

• WHAT
  • Fix FilesExt's upload and download should fallback when Presigned URLs are not available in certain compute types.
• WHY
  • In some newer compute clusters (e.g. model serving clusters), creating presigned URL through Files API is not possible yet. Client should fallback in such circumstances to not using Presigned URL. Or else the customer wouldn't be able to upload or download data using w.files.

How is this tested?

It is tested using unit tests.

[github-actions]

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Trigger:
go/deco-tests-run/sdk-py

Inputs:

• PR number: 1093
• Commit SHA: 3e419918e73209e4de23138ad5607eb557488d33

Checks will be approved automatically on success.