Upload rejection of a file of forbidden type breaks 'Close' in 'Save & Close'

To reproduce

• Configure the system to only allow upload of files with specific extensions.
• Have a document type with a field allowing to upload 'embedded assets'.
• Create a document of that type and have it opened for editing.
• In the CMS document edit screen, try to upload a file with extension on on the configured list of permitted extensions.
• Observe that:
  • correctly:
    • Upload is denied.
    • Error message gets displayed, explaining the reason for refusal.
    • 'Save & Close' button saves any outstanding changes (obviously, this will only become apparent when the document gets reopened later).
  • incorrectly:
    • 'Save & Close' button does not close the document.

Workaround 1

• Click the little 'x' icon on the tab in the bottom right corner, the one with the document name (the same one that allows you to dismiss unsaved changes).

Workaround 2

• Log out,
• Log back in,
• Click 'Save & Close' - this time it will work.