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@feld both topics remain under discussion FWIW, and have some prospective implementation path. Large files could be transferred via @n0iroh which is already integrated but this is only provides for the start of a discussion that we typically don't entertain in public but in our nested maze of chat groups and in real-life gatherings ...
as to link-previews, it's likely we generat them on the sender-side so there shouldn't be rate-limits or privacy issues (as opposed to a bot solution).