Resolve "Follow-up from "Implement skynet directory upload""

MERGE REQUEST

MR Guidelines

Overview

Main points:

• remove a no-op return in skynetUploadDirectory and make it properly stop the execution
• move the helper function AddMultipartFile to a more appropriate place

Also:

• as I'm already touching some test files, there's no harm in fixing some minor issues I stumble upon, right?

Example for Visual Changes

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created

Issues Closed

Closes #4382 (closed)