One of the new things I’ve seen at the workshop in Berlin, was the ability to modify informations shown in the panel. That is (like most of Kirby) new to me.
The F class provides basic file handling stuff. Methods from this class are used within the File class.
Note that if you do something with your file, like rename it in the Panel etc., the modification time will change.
If you really only want the upload timestamp and make sure that this date is not modified by the file system if you move stuff around, I’d store the date in the meta data on upload using a hook.