Change extension for CSV data

[axel584]

Description

When I download data in text / csv format, I would like the extension offered to be ".csv" (instead of the ".txt" currently offered)

Motivation and Context

Currently, the extension of the file is ".txt" and users seem to have difficulty opening the generated file.

How Has This Been Tested?

I try the fix on my local swagger.

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

I am not sure if the current name used at the time of data download is covered by a unit test.
The current code manages several formats (image, pdf, xml, html ...). If this part of the code is covered by unit tests, they should indeed be added.