File list input with default value returns "Invalid upload file" when accessed through Web App

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### Dify version

1.17.0

### Cloud or Self Hosted

Cloud

### Steps to reproduce

1. Create a blank Chatflow application.
2. Add a File List input field, set a Default Value, and uncheck Required.
3. In the LLM node, enable Vision and set the variable to the one created in step 2.

### ✔️ Expected Behavior

The file list with the configured default value should be readable through both Preview and WebApp.

### ❌ Actual Behavior

The file list works correctly through Preview, but returns Invalid upload file when accessed through the WebApp

Preview

WebApp

Reproduce the issue with a blank Chatflow, an optional File List input with a default value, and Vision enabled in the LLM node. Compare the Preview and WebApp paths and trace where the WebApp rejects the configured file list. Done means the default file list is readable in both Preview and WebApp without an "Invalid upload file" error.

Written by the indexing model from the issue text.