OK so for that you should save file with UserId appended.
Example if FileName is myFile.txt Save it as myFile_1023.txt where 1023 is the UserId
For that file saving you will need to do
FileUpload1.PostedFile.SaveAs(Server.MapPath("~/Uploads/") + Path.GetFileNameWithoutExtension(FileUpload1.PostedFile.FileName) + "_" + 1023 + Path.GetExtension(FileUpload1.PostedFile.FileName));
Now to get Files for specific User i.e. 1023
string[] filePaths = Directory.GetFiles(Server.MapPath("~/Uploads/"), ""*1023*".*);