Fix Teleporter file selection on Mac OS

[DL6ER]

What does this implement/fix?

Use MIME type instead of explicit file type endings during the file selection of the Teleporter input field. Even when the latter should be fine according to the standards, Safari (and, in general, any browser on Mac) seems to ignore this and support only mimetype-based selections. This is backed by several similar reports on Stackoverflow over the past couple of years.

I am not convinced this is the best way of implementing this - in particular because we are not in the wrong here - Mac OS is clearly not complying with standards here - and this is a mere compromise to please them.

Related issue or feature (if applicable): See link below.

Pull request in docs with documentation (if applicable): N/A

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[PromoFaux]

I don't have a mac to test this against, but still working on Windows so....