I don’t see discussion of how to do it for hosted in meta discourse…
If you’re a CDCK hosted customer I would recommend asking this question via their support system, as this almost certainly isn’t something you can do yourself.
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Just a note on this for an update, if you’re on our hosting 
you can now adjust both max_image_size_kb and max_attachment_size_kb from the admin settings up to 30MB (30720 KB) and it should JustWork™. 
There’s now no need to contact @team for any backend changes. 