How to inhibit URL-preview?

When pasting an URL in the textfield, sometimes previews are embedded automatically, does someone know, how to control that behaviour/can a user decide if, or not, a link-dest is embedded?

Seems the preview happens only with github-links. Questions are, if we want this (I don't) and where it's configured?
A quick search gave me https://github.com/discourse/onebox is delivering this feature.
For embedding videos and other media it could be useful, though.
Luxurious would be a config-option for each user via prefs.

In the moment this is not of urgency, until enabling category-distinction in emails is solved.

Even i want to know the answer on this.

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