Add warning before file upload

[philip-cline]

Checklist

• [x] Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
• [x] Any modified or new methods or classes have helpful JSDoc and code is thoroughly commented
• [x] The description lists all applicable issues this PR seeks to resolve
• [x] The description lists any configuration setting(s) that differ from the default settings
• [x] All tests and CI builds passing

Description

Display the external merge warning before and after file upload.