## Summary
This PR adds support for attaching files to agent chat messages. Users
can now upload files when sending messages to the AI agent, and these
files are properly processed, stored, and made available to the agent
with signed URLs.
## Key Changes
- **File attachment input**: Added `fileIds` parameter to the
`sendChatMessage` GraphQL mutation to accept file IDs from the client
- **File processing**: Implemented `buildFilePartsFromIds()` method to
convert file IDs into file UI parts with signed URLs
- **Message composition**: Updated user messages to include both text
and file parts when files are attached
- **File URL signing**: Integrated `FileUrlService` to generate signed
URLs for files in the AgentChat folder, ensuring secure access
- **Message persistence**: Files are now included in the message parts
stored in the database and retrieved when loading conversation history
- **File metadata mapping**: Enhanced `mapDBPartToUIMessagePart()` to
properly extract MIME types from file entities and include file IDs
## Implementation Details
- Files are fetched from the database using the provided file IDs and
workspace context
- Each file is converted to an `ExtendedFileUIPart` with proper metadata
(filename, MIME type, signed URL, and file ID)
- When loading messages from the database, file parts are enhanced with
signed URLs to ensure they remain accessible
- The `loadMessagesFromDB()` method now requires the workspace ID to
properly sign file URLs
- File attachments are seamlessly integrated into the existing message
part system alongside text content
https://claude.ai/code/session_01TAdN1gBzeiYELX4XDrrYY1
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Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
