1-Make sure that there is a commons-fileupload.jar file in your WEB-INF/lib directory and make sure it's the one that came with the same Struts download as your struts.jar file.
2-Make sure that you don't have any commons-xxx.jar files in common lib directories used by all applications. This would cause it to be loaded by a different ClassLoader than strus, and could cause problems.
If you've verified the above and are still getting the error, post your JSP code (at least the <html:form> tag and the <html:file> tags) as well as the relevant portions of your struts-config.xml file and your ActionForm bean. We'll then have more information to help you solve this.