I've tried to post a bug report but there is no 'Create Ticket' button.
In efax-gtk 3.2.12-1 the buttons for 'preview' and 'ok' are interchanged -
[ok] opens the choosen file in gv, [preview] adds the file in the text field 'file to send'.
I guess you are probably using arch. If arch has moved to GTK+-3.12, this broke the file chooser dialog in efax-gtk. This has been fixed in efax-gtk-3.2.13. I suggest you ask the maintainer to update the version of efax-gtk used by arch.
As a side issue, it looks as if arch may be using a patched version of GTK+-3.12, because with efax-gtk-3.2.12 and the vanilla GTK+-3.12 code, the file chooser dialog fails to do anything when the OK and View buttons are pressed. This is because in GTK+-3.12 the buttons of GtkDialog objects have been made internal instead of non-internal children of the action area box, so they are inaccessible. Possibly this has been patched in arch but the buttons are now in the wrong order.
On second thoughts, I think what has probably happened is that arch's efax-gtk-3.2.12 binary was originally compiled against GTK+-3.10, and the ABI changes in GTK+-3.12 concerning GtkDialog's action area have the side effect of causing the wrong button callback to be executed. If you recompile efax-gtk-3.2.12 against GTK+-3.12 you will probably find that the buttons no longer work at all.
As mentioned, the correct thing to do is to upgrade to efax-gtk-3.2.13.