Files tab: Cleared file is added to the document

Steps to reproduce:

1. Go to the Files tab of a document.
2. In the Upload widget, click on Add and select a file.
   -> File is displayed in the selection in the widget.
3. Remove the file from the selection by clicking on the Clear link of the file or the Clear all button.
   -> Selection is empty.
4. Click on Add and select another file.
   -> 1 file (new one) is displayed in the selection.
5. Click on the button Store uploaded files.
   => 2 files are saved as attachment to the document.