Add UI capability to remove attachments once added

The way the "Add Attachment" UI component can be used is different across browsers.

• Firefox shows the path in a text box, but it can't be modified
• IE shows the path in a text box, but it can be modified
• Safari shows no path

IE is the only browser which allows you to "cancel" the adding of an attachment, by blanking out the text box which contains the path. For other browsers, once you have chosen a file with the UI component, you must upload something (either the file chosen or another one).

Perhaps we could add a UI widget that removes the component from the form, so that the file is not uploaded.