Unfortunately, I'm passing this ticket on to you second-hand so please bear with me.
Since at least version 2.3.3 we have had a problem with Struts and multipart/form uploads. The problem is intermittent and we'd had a really hard time getting it to occur consistently on all our servers, but it seems to happen pretty reliably on Red Hat.
The problem is basically this: When a multipart upload is received by Struts, the action responsible for the request is instantiated without any information about the upload. We expect to get the file name/type/content as per: http://struts.apache.org/release/2.3.x/docs/file-upload.html but get null values instead.
We were able to track the problem back into the org.apache.struts2.dispatcher.Dispatcher a while back and circumvented the issue as you can see in the attached patch ... but we don't know enough about Struts to figure out if a fix should be applied elsewhere.
It's entirely possible we're using Struts wrong, but we tried pretty hard to find the right upload configuration and couldn't get reliable results until we patched.
Please let me know if need more information to reproduce the problem. Our CMS is behind a firewall, so I can't point you to a live page but would be glad to give you more details about the JARs we use and how we have Struts configured.