Fix File Upload widget on change when a new file is not chosen

1. install 7.10
2. create a workspace and a document of type File
3. attach a file to it
4. click Edit
5. in the attachment select Content / Upload but don't choose a file.
6. click Save
7. the field Content is now empty and does not have the Keep option file in any longer whereas text fields do.
8. the only solution is then to click the Summary tab and then Edit again, which makes the user lose all the other values entered

Expected behavior: the value should be kept.

Can be reproduced on Firefox, Chrome and Safari (not tested with IE).

Tested with latest 7.10-HF20 -> no change.