v1.16 Agent mode: uploaded files not displayed in UI and file variables invalid in build

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### Dify version

1.16.1

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

1. Go to Agent app → click Build (构建) to enter the "Build Agent via Conversation" interface.
2. Upload any supported file (e.g., a PDF) via the attachment button in the chat input area.
3. Type a prompt referencing the uploaded file, e.g., "Can you see the file I uploaded?"
4. Click "Start Build" (开始构建).
5. Observe the Agent's response.

### ✔️ Expected Behavior

The uploaded file should be transferred into the Agent's sandbox workspace.
The Agent should be able to read and process the file content to build the Agent configuration.

### ❌ Actual Behavior

The file appears successfully uploaded in the UI (file name and size are displayed).
After clicking "Start Build", the Agent responds: "I currently do not see the file you uploaded. I just checked the sandbox workspace for this run... In other words, you have not passed the file over in this turn."
The file is not present in the sandbox workspace at all.
This issue persists across Build, Preview, and Published modes — uploaded files are never accessible by the Agent.

Start with the Agent app's Build via Conversation attachment flow and trace how an uploaded file is passed into the sandbox workspace. Compare the behavior across Build, Preview, and Published modes; done means the uploaded file reaches the workspace and the Agent can read and process it in each mode.

Written by the indexing model from the issue text.