Fill in the Input File field with a file name other than
  the one located in your default directory.

  After the reserved generation is replaced, CMS deletes
  the file used to create the new generation (and any
  earlier versions of the file in the same directory)
  unless you turn off the Delete files when replaced
  button, or turn on the Reserve Again button.

  If you do not supply a file name, CMS searches your
  current default directory for a file with the same name
  as the element.