feat: support multiple file upload in Attachments component (#13283)

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)


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Let me know if you'd like any changes! 💪

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Co-authored-by: Félix Malfait <felix@twenty.com>

packages/twenty-front/src/modules/workflow/workflow-steps/workflow-actions/ai-agent-action/components/AIChatTab.tsx

@@ -85,7 +85,7 @@ export const AIChatTab = ({
       {isDraggingFile && (
         <DropZone
           setIsDraggingFile={setIsDraggingFile}
-          onUploadFile={(files) => uploadFiles([files])}
+          onUploadFiles={uploadFiles}
         />
       )}
       {!isDraggingFile && (