fix: matplotlib images are loaded again in the frontend

[AnnaPolensky]

Description

fixes #441
Same mistake as #421
The frontend expected a different response format from views.py for images.

Testing

Create a workflow like this and see that the image is rendered. Use Reactome_enrichment_enrichr_2022 from enrichment_data for the upload.

PR checklist

Development

• If necessary, I have updated the documentation (README, docstrings, etc.)
• If necessary, I have created / updated tests.

Mergeability

• main-branch has been merged into local branch to resolve conflicts
• The tests and linter have passed AFTER local merge
• The backend code has been formatted with black
• The frontend code has been formatted with pnpm format and checked with pnpm lint

Code review

• I have self-reviewed my code.
• At least one other developer reviewed and approved the changes

[jorisfu]

👍 💯 🥇 🎊