Initialize wxImageFileProperty correctly when file is given in the ctor.

Previously the file had to be specified later and the value passed to the ctor
was ignored.

Closes #15456.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74729 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775