I think the problem has more to do with the FileUploadPlugin returning an array containing entries for all form field items, not just the file inputs. Also, the FileItem getFile(FormFieldName) method in the plugin is not exposed in the FileUploadPlugin API, so I had to resort to getting the whole List<FileItem> and iterating through to finding one with a matching field instead of assuming I knew the correct index. I'm keeping my fingers crossed that the XWiki folks don't start using one-based arrays </shudder>