[Feature] Batch download for selected images and videos via Telegram selection mode

[Ericcwang23]

Related

• Closes Will the batch download feature be updated in the future? #75

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Background

This feature adds support for batch downloading images and videos from selected messages in Telegram WebApp.

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Proposal

Introduce a batch download workflow based on Telegram's native message selection mode.

The system uses user-selected messages as the source of truth for media collection.

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User Flow

1. User enters Telegram's native selection mode (multi-select UI)
2. User selects one or more messages containing media
3. A contextual action button appears in the chat header:
   • 📥 Batch Download (N media / M messages)
4. User triggers the download action
5. Selected messages are processed sequentially
6. Button is hidden when selection mode is exited

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Scope

• Platform: Telegram WebApp (/a/)
• Supported media types:
  • Images
  • Videos
• Album support: grouped messages
• Download strategy: sequential processing with throttling (e.g. 300–500ms delay)
• Fallback: blob URL based download (no dependency on native file picker APIs)

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Implementation Notes

• Selected message IDs are used as the input source
• Media items are extracted from selection context
• A download queue is processed sequentially to prevent request bursts
• UI state is bound to selection mode lifecycle events

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Reference Implementation

A working prototype has been implemented locally for validation purposes:

https://github.com/Ericcwang23/Telegram-Media-Downloader

This is provided as a reference implementation of the proposed workflow.