Here is a link to the same question posed in another forum. The respondent shows a clever way to do this. I haven't tried it, but you might want to give it a go.
Since you're using Struts, you could replace their line:
<input type="file" name="browse" style="display:none;">
with
<html:file property="theFile" style="display:none;" />
Just make sure you replace all references to "browse" in their javascript with "theFile".