Posting the same HTML-Form with or without a single stripes:file parameter affected whether stripes:text-RequestParameter-StringProperties were injected into the ActionBean with right or wrong character encoding (German Umlaute). After some debugging I found out that net.sourceforge.stripes.controller.multipart.CommonsMultipartWrapper does:
[...]
for (FileItem item : items) {
                // If it's a form field, add the string value to the list
                if (item.isFormField()) {
                    List<String> values = params.get(item.getFieldName());
                    if (values == null) {
                        values = new ArrayList<String>();
                        params.put(item.getFieldName(), values);
                    }
                    values.add(item.getString());
                }
                // Else store the file param
                else {
                    files.put(item.getFieldName(), item);
                }
            }
[...]

in its build method. I changed

      [...]
      values.add(item.getString());
      [...]

to

      [...]
      values.add( (request.getCharacterEncoding() != null) ? item.getString(request.getCharacterEncoding()) : item.getString() );
      [...]

and everythink works fine for me.