Accessing properties of attached file
Partial answer, but in the views settings page, you add a relationship to the field that holds the file.  Then, when you add a field, you should see an option for File: Size in the list.  Other fields for file into appear, but mtime isn't one of them.

A hackish way to get mtime is to use stat in your view template, but I would do that as a last resort if you can't find a module to add this as a view field.