solved the problem!
Had an normal<h:form... element on the jsp-page. Closed the form-element above the fileupload - component and surrounded the fileupload-component with <a4j:form> . Now the content is transmitted properly. Don't know why, but funzt.
Can you please provide some more information on how to reproduce the issue: web.xml, full page code, server and JSF versions? Thank you in advance!