Add allowed_file_extensions arg to file_upload
Add allowed_file_extensions arg to file_upload

Safari has a bug where it stops input[type=file]
elements working if they don't specify the types
of file to accept (via the `accept` attribute).

It seems to just effect certain versions of Mojave
but completely blocks this action so worth fixing.

This adds a 'allowed_file_extensions' keyword
argument to the file_upload component to let you
specify a value to be passed to `accept`.

This was spotted on x-gov Slack:

https://ukgovernmentdigital.slack.com/archives/C06GCJW7R/p1607952390112800

...and StackOverflow:

https://stackoverflow.com/q/64843459/679924

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