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Adding a link where the content is hosted, instead of the content directly? Def. possible - even after the creation of the post. But it would only make sense if frontends would support this option before the content is actually created and transaction broadcasted to the blockchain.

I wonder if the other frontends would honor it

I guess it depends on the usage.

It would need to be peer to peer I imagine since hosting on a server or cloud could be both costly and prone to attacks.

That's a good point.