Add missing methods for FormData type

[NColey]

Fixes #14813.

[msftclas]

@NColey,
Thanks for your contribution.
To ensure that the project team has proper rights to use your work, please complete the Contribution License Agreement at https://cla.microsoft.com.

It will cover your contributions to all Microsoft-managed open source projects.
Thanks,
Microsoft Pull Request Bot

[msftclas]

@NColey, thanks for signing the contribution license agreement. We will now validate the agreement and then the pull request.

Thanks, Microsoft Pull Request Bot

[mhegazy]

Thanks for your PR. these files are not manually edited, they are auto-generatd from a script in . You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TSJS-lib-generator.

Changes should be done in https://github.com/Microsoft/TSJS-lib-generator instead.

Please find more information at: https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[msftclas]

@NColey,
Thanks for having already signed the Contribution License Agreement. Your agreement has not been validated yet. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[mhegazy]

Your changes to src/lib/DOM.iterable.d.ts we should take, please revert the other changes and send them to TSJS-lib-generator instead.

[NColey]

Hi @mhegazy I reverted the unnecessary changes leaving just the updates to src/lib/DOM.iterable.d.ts and moved the rest to the following PR in TSJS-lib-generator. Let me know if there is anything else I should do, thanks!

[mhegazy]

from spec, the values are string | File; that applies to value, entries and [Symbol.iterator].

[NColey]

I just updated the PR with the appropriate types, thanks!

[mhegazy]

thanks!