Provide a way to differentiate unfurled media owned by the application from other Discord CDN URLs

[freya022]

Context

This post will focus on Components V2

I believe libraries should be able to automatically retain attachments when editing messages, this will go toward an effort to:

1. Improve the developer experience (no manual attachment management)
2. Improve the user experience (faster response times)
3. Reduce the bandwidth costs for Discord

The issue

To implement this, one would currently have to parse the unfurled media URLs, to obtain the attachment ID which is the 2nd snowflake.

However, this method falls apart when an app starts using URLs of attachments they don't own, as Discord will return an ATTACHMENT_NOT_FOUND error response.

Proposal

The goal is to provide library developers the necessary tools to retain attachments they do own, here is a few ideas:

• Add a was_uploaded field to unfurled media, if the app has permissions to edit then surely they can also retain attachments whose was_uploaded value is true
• Add a nullable application_id field to unfurled media, this way an application can check if it corresponds to them. If I'm not mistaken, this should also be set for webhook messages owned by applications.

[DV8FromTheWorld]

I have implemented attachment_id as a field on UnfurledMedia when they are backed by an attachment via the attachment://<filename> protocol. You should now be able to know what attachment ids are in use on the message and use those when sending the attachment array to properly retain them.