fix: can send FormData with File.

[sainthkh]

• Closes Posting formData using cypress doesn't work #1647

User facing changelog

As socket.io ignores FormData body, Cypress driver couldn't send FormData with file. This PR fixes this behavior.

Additional details

• Why was this change necessary? => Sending files is a valid behavior, but cy.request() doesn't support it.
• What is affected by this change? => N/A
• Any implementation details to explain? => This PR encodes FormData to base64 string to bypass socker.io.

How has the user experience changed?

N/A

PR Tasks

• Have tests been added/updated?

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[sainthkh]

Flaky failures I guess.

[chrisbreiding]

I think the 2 tests currently failing are legitimate. Looks like something is causing socket-io's hasBinary to infinitely recurse.

[sainthkh]

@chrisbreiding I failed to reproduce the hasBinary flakiness. And I also found that it exists in the develop branch (check #16640). It is maybe introduced from my previous PR, #15931. It's caused by circular object/array sent through socket.io. I'm trying to research when a circular object is passed to the server and why.

[chrisbreiding]

I saw the hasBinary failures a couple times in a row, so I thought it was consistent, but you're right that it's flakiness. I appreciate you looking into the cause of it.