Don't immediately send attachment on upload

[WesleyAC]

When a user uploads a attachment or image, it's immediately sent. Instead, we should change the behaviour to be more like the webapp: the input field should have the markdown for displaying the image inserted, but the user should be allowed to edit before sending. This will also allow people to add multiple attachments (see this CZO thread requesting this).

Related: #1903

[vaibhavs2]

can it be assigned to me :)

[chrisbobbe]

Also related: #2684