blobs: derive missing contentType from file extension

[alxndrsn]

Ref #1351

What has been done to verify that this works as intended?

Updated tests.

Why is this the best possible solution? Were any other approaches considered?

Discussed on #1351

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Replace Content-Type response header of string "null" with text/csv; charset=utf-8 for .csv files, and application/geo+json for .geojson files.

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

Yes, done.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced