mvc dropdownlist所选项目无法重选

In a view (UserPicture.aspx), I have a dropdownlist of users, an image, a file upload and a submit button. When I change the selected user, I want to change the displayed image to the corresponding profile picture. Every thing works fine until I modify the image for a user, using fileupload and submit button. After that, if I try to reselect the changed user, In a view (UserPicture.aspx), I have a dropdown