Class
An item that can be used in a Dialog to allow the user to upload a file. It creates HTML content with two DIVs, one that permits dragging and dropping of files onto the dialog to upload them, and the other of which contains a button you can click to choose a file.
Whenever the user chooses a file, the file's information are stored in the
uploadedName and uploadedContent properties of the object, and if the
instance implements the .onFileChanged() method, that method is called.
Construct an upload control/area, as described above.
string
the key to use to identify this input control's content in the dialog's key-value mapping for all input controls