fix: update file upload to use RC 8.x rooms.media API

[dodaa08]

• Replace deprecated rooms.upload endpoint with new two-step process
• Step 1: POST to rooms.media/:rid to upload file
• Step 2: POST to rooms.mediaConfirm/:rid/:fileId to confirm with metadata
• Fixes file upload 404 error on Rocket.Chat 8.x servers

Brief Title

fix: update file upload to use RC 8.x rooms.media API

Acceptance Criteria fulfillment

[x] File uploads work on Rocket.Chat 8.x servers [x] Thread attachments supported (tmid parameter) [x] File description is properly sent [x] Auth headers included in both requests

Fixes #1069

Video/Screenshots

https://github.com/user-attachments/assets/8d4ad2f5-d174-4fb0-89d7-3f8328832fd5

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-<pr_number> after approval. Contributors are requested to replace <pr_number> with the actual PR number.