Add Content-Type and Content-Disposition for plain text, json, and PNG

[lidel]

Content-Type and Content-Disposition header in response have high impact for web browser experience.
They inform browser how to render content and what filename is used in "Save As" user flows.

Ref.

• https://specs.ipfs.tech/http-gateways/path-gateway/#cache-control-response-header
• https://specs.ipfs.tech/http-gateways/path-gateway/#content-disposition-response-header
• "Save as" broken: saves SW bootstrap HTML instead of IPFS data service-worker-gateway#574 (we want to avoid bugs like that)

TODO

Some of below may already exist, but we should confirm and fill any gaps:

Content-Type test for

• plain text
• JSON
• image (1x1px PNG?)
• raw block
• car
• ipns-record

Content-Disposition test for

• plain text
• JSON
• image (1x1px PNG?)
• raw block
• car
• ipns-record