Mobile image caption (alt text) not displayed on desktop/web clients

Description:

When a user sends an image with a caption from the mobile app,
the caption text is not visible on desktop clients (Windows/Linux/Web).

Steps to reproduce:

1. Open Rocket.Chat on mobile
2. Send an image with a text caption
3. Open the same channel on desktop app or web browser
4. The caption text is missing

Expected behavior:

Caption text should be visible below or above the image.

Actual behavior:

Caption text is only stored in the alt attribute of the <img> tag
and is not rendered as visible text on desktop.

Root Cause (HTML analysis)

Mobile message — text only in alt, no rcx-message-body block:

Mobile User IMG_6827.jpg (48.76 kB)

Desktop message — text in separate rcx-message-body block, alt is empty:

Desktop User Text sent from desktop as separate message body Clipboard - screenshot.png (21.94 kB)

Server Setup Information:

• Version of Rocket.Chat Server: 8.4.0
• License Type: Starter
• Number of Users: 41
• Operating System: Rocky Linux 9.7
• Deployment Method: docker
• Number of Running Instances: 1
• DB Replicaset Oplog: Not configured
• NodeJS Version: v22.16.0
• MongoDB Version: 8.0.21 / wiredTiger

Client Setup Information

• Desktop App or Browser Version:
• Operating System: Ubuntu 24 - 4.14.1, Windows 11 - 4.14.0, IOS 26.5 - 4.72.0.108804, Android 15 - 4.72.0.108809

Additional context

Relevant logs:

Start by reproducing the mobile image-caption case in the desktop or web client and trace the message rendering path for uploaded images. Compare the mobile-generated markup, where the caption is only in img alt, with the desktop markup, and verify that the caption becomes visible text without losing image accessibility.