fix: Image rendering in the knowledge base failed.

[FFXN]

Important

1. Make sure you have read our contribution guidelines
2. Ensure there is an associated issue and you have been assigned to it
3. Use the correct syntax to link this PR: Fixes #<issue number>.

Summary

fixes #35910

In the environment variable configuration INTERNAL_FILES_URL, the value is the API service address in the container, such as http://dify-api-svc:5001/.
Upload images for retrieval during the knowledge base retrieval test.
Then the browser will not request the image asset.

Screenshots

Before	After
...	...

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Diff

No changes detected.