Ajax form submission does NOT occur if it contains a struts2-file tag (the form gets submitted and expected content correctly shows up in the target div IFF the file tag is removed). Code snippet below where I've prefixes, 's','sx' defined for 'struts-tags','struts-dojo-tags' respectively and 'sx:head' inside <head> tag.
Struts2 versions: (struts 2.1.8,struts2-dojo-plugin-2.1.6.jar).
I tried the same few months before and at the end of the day I came to above conclusion. At last I did it by using frames (Even though it looks like ajax file upload, But technically it is not).
Also please use code tags. And welcome to java ranch