Rationalise prepareUpload methods

[willp-bl]

What problem does the pull request solve?

Now only one (internal) prepareUpload exists. Note this doesn't change the external API. The new internal method uses a boxed boolean (Boolean) so it can be null when called from the existing prepareUpload() methods.

Checklist

• [x] I’ve used the pull request template
• [ ] I’ve written unit tests for these changes
• [ ] I’ve updated the documentation in
  • [ ] notifications-tech-docs repository
  • [ ] CHANGELOG.md
• [ ] I’ve bumped the version number
  • [ ] in src/main/resources/application.properties
  • [ ] in pom.xml