Add download_attachment functionality to extract email attachments

[gsdali]

Summary

• Adds download_attachment method to the base EmailClient class and implements it for both Gmail and Outlook providers
• Adds new /download_attachment API endpoint
• Allows downloading attachments by message ID and attachment index
• Supports both saving to disk and returning base64-encoded data

New Endpoint

GET /download_attachment?msg_id=<id>&attachment_index=0&save_dir=/path/to/save

Parameters:

• msg_id (required) - The email message ID
• attachment_index (default: 0) - Which attachment to download (0-based)
• save_dir (optional) - If provided, saves file to disk; otherwise returns base64 data

Response:

{
  "operation_status": "succeeded",
  "operation_message": "Attachment has been downloaded successfully",
  "result": {
    "filename": "document.pdf",
    "mimeType": "application/pdf",
    "size": 249216,
    "total_attachments": 2,
    "saved": true,
    "filepath": "/path/to/save/document.pdf"
  }
}

Test Results

Test	Result
Download as base64	✅ Works
Save to disk	✅ Works
Multi-attachment email (index selection)	✅ Works
Invalid index error handling	✅ Works
No attachments error handling	✅ Works

Files Changed

• email_client/BaseEmailProvider.py - Added abstract method + success message
• email_client/gmail_helpers.py - Gmail implementation
• email_client/outlook_helpers.py - Outlook/Graph API implementation
• api/main.py - New endpoint

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