Looking at the usage guide on extundelete it seems as though you're limited to undeleting files to a few ways.
extundelete is designed to undelete files from an unmounted partition to a separate (mounted) partition. extundelete will restore any files it finds to a subdirectory of the current directory named “RECOVERED_FILES”. To run the program, type “extundelete --help” to see various options available to you.
Typical usage to restore all deleted files from a partition looks like this:
$ extundelete /dev/sda4 --restore-all
Restoring a single file
In addition to this method highlighted in the command line usage:
Attemps to restore the file which was deleted at the given filename,
called as "--restore-file dirname/filename".
So you should be able to accomplish what you want doing this:
$ extundelete --restore-file /var/tmp/test.iso /dev/sda4
NOTE: In both cases you need to know the device,
/dev/sda4 to perform this command. You'll have to remount the filesystem as readonly. This is one of the conditions of using
extundelete and there isn't any way around this.