Remove # from channel name when uploading file

Testing done

see https://github.com/jenkinsci/slack-plugin/issues/884#issuecomment-2098212808

### Submitter checklist
- [ ] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [ ] Ensure that the pull request title represents the desired changelog entry
- [ ] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

jenkinsci / slack-plugin

Remove # from channel name when uploading file #962

Testing done