Closes#13277
## What I Did
- Enabled `multiple` attribute in the file input
- Updated the `handleFileChange` handler to loop through files
- Confirmed that each file is sent via `uploadAttachmentFile`
## Why
This change allows users to upload multiple files at once instead of
doing it one by one.
## Screenshot / Video (Optional)
[Screencast From 2025-07-18
23-58-36.webm](https://github.com/user-attachments/assets/ea191f25-1904-4643-afe2-7029785eebcb)
---
Let me know if you'd like any changes! 💪
---------
Co-authored-by: Félix Malfait <felix@twenty.com>
