org-download-heading-lvl with German umlaut

[fab6]

Hi,

I am using these settings to get screenshots into directories fitting to the header title:

(setq org-download-method 'directory)
(setq-default org-download-image-dir "./img")
(setq-default org-download-heading-lvl 1)

This works quite good!
As I am writing often in German it happens that I have German umlaute in the title. This creates the screenshot and puts it in the correct directory, but I am not able to preview it, e.g. like here

When I change the umlaut ü to ue the preview works good again.

Is there an option which automatically replaces the different umlaute (ü,ä, ö) before saving?
Or do you have some other hint for this?

Thanks!