feat: support adding attachments when editing messages

[Minh141120]

Problem Statement

Currently, when editing a message, users can only remove existing attachments but cannot add new ones.

This limitation creates friction for users who:

• Forget to include an attachment when first sending a message.
• Realize after the fact that an image, screenshot, or file would provide important context.
• Want to enrich a discussion without creating duplicate or follow-up messages.

As a result, conversations may become fragmented, and users often resort to posting additional messages rather than keeping context in one place.

Feature Idea

Extend the existing edit flow to allow users not only to remove but also to add and update attachments.

[Vanalite]

Seems like this is not really, users can stop the previous generating response and add image/attachment to the next message.