Maintains a list of {@link UploadDetails} ands allows the upload systemto access them via their id.
In general, a user would request an upload. The controlling action would then create a new upload details object and add it to their sessions upload manager. Control would then be passed to {@link ShowUploadAction}given it the upload id.
In general, you would never need to create an instance of this class yourself and should instead use {@link CoreUtil#addUpload(javax.servlet.http.HttpSession,UploadDetails)}.