Fixed type of event parameter in FileReader callbacks

[acsbendi]

Fixes #4163 which is closed already but was in fact only partially fixed. The invalid type of the event parameter in FileReader callbacks is still causing issues like this

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[typescript-bot]

It looks like you've sent a pull request to update our 'lib' files. These files aren't meant to be edited by hand, as they consist of last-known good states of the compiler and are generated from 'src'. Unless this is necessary, consider closing the pull request and sending a separate PR to update 'src'.