text: Title: [BUG] dcc.Upload does not support re-upload the same file.
  Body: It appears that if one uses the dcc.Upload.contents property for triggering callback, it fails when the user tries to upload the same file again, due to the content is the same.
  
  I would imagine that a property similar to `n_clicks` should be provided to trigger callback on each upload event.
  
  Versions:
  ```
  dash                      1.12.0
  dash-core-components      1.10.0
  dash-html-components      1.0.3
  dash-renderer             1.4.1
  ```