multifilesink: post file message on EOS

multifilesink: post file message on EOS

When multifilesink is operating in any mode other than one file
per buffer, the last file created won't have a file message posted
as multifilesink doesn't handle the EOS event.

This patch fixes it by using the last position to post a file
message when EOS is received. This should ensure at least the
time related data and the filename are posted to the application
or other elements

https://bugzilla.gnome.org/show_bug.cgi?id=747000