Sharing from outside Keybase iOS client doesn't honor exploding chat messages unless chat retention policy is set

I noticed that some conversations where I have exploding messages set had old images/videos retained past the expected exploding time. Seems that items shared from outside the Keybase client using the built-in iOS sharing option and selecting Keybase are posted with no exploding timer set. If the same items are shared from within the Keybase app, they pickup the timer correctly. Regardless of how the items are shared, if the overall chat retention policy is set, the items are still deleted according to that policy. But it seems like they should also be picked up by the per-chat post settings of each user.

Start by reproducing the issue through the iOS built-in sharing option and compare it with sharing the same item from within the Keybase app. The work is done when externally shared images and videos honor each user's per-chat exploding-message setting, including when no overall chat retention policy is configured.

Written by the indexing model from the issue text.