FileObject: add a method to determine the file name as per a given date
FileObject: add a method to determine the file name as per a given date

We're about to add a command-line method to list the history as per the
current set of update sites. To make this as informative as possible,
let's use the correct file names.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

imagejan/imagej-updater