Dispatch event so listeners would see changed file

[yurynix]

Closes #5420

In a similar way it's done at:
https://github.com/puppeteer/puppeteer/blob/master/lib/JSHandle.js#L305

We should notify listeners the input is changed.

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[yurynix]

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[gziolo]

Just noting that there is a similar patch approved: #5389. I hope it's merged and a new version published to npm sooner than later. At the moment, it prevents upgrade to the latest version for projects that have media upload in their specs.

[mathiasbynens]

Thanks! Closing in favor of #5389, which was merged.