Allow file upload to select same file after error

[alefesouza]

Fixes #460

Changes proposed in this Pull Request

• Fixes an error that occur when a user upload a file and have some connection or general error, if select the same file again it does not emit the onChange event and the user get stuck until select a different file.

Needed to use a workaround to empty the file input because WordPress/gutenberg#39268 will be implemented on the next Gutenberg release and WCPay still uses the version 5.1.2 of it.

Testing instructions

• Buy something on a merchant store and add a payment that generates a dispute.
• Access the Transactions section on the left panel and open the latest transaction, if it does not work (this page is always empty to me), open the latest order and click on the "pi_hash" link.

• On this page there is a "View dispute" link, click on it.

• Click on "Challenge dispute".
• Change the "Product type".
• The file input button will be on this page.

• Now we need to make an error occur, to do it, just select a file that is not a PDF/JPG/PNG file.
• Now select the same file again, it will show the loading animation, before this PR the user would get stuck because the onChange event was not called.
• You can also disable the internet on the browser DevTools and try to upload a file, enable the internet again and selected the same file, before this PR the "You are offline" message would be showing until the user select another file.

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• Run npm run changelog to add a changelog file, choose patch to leave it empty if the change is not significant. You can add multiple changelog files in one PR by running this command a few times.
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Link to testing instructions from release testing doc : Add link here / 'QA Testing Not Applicable'

[brianyu28]

This looks good to me, thanks @alefesouza! I left one suggestion about leaving a comment documenting the relevant Gutenberg issue, otherwise this looks good and works well.

[brianyu28]

I'd suggest leaving a comment here that links to WordPress/gutenberg#39267, so that it's clear why this workaround is being used. That'll also make it clearer later when we might want to use onChange directly on FormFileUpload once it's supported.