Certainly this "iCloud upload being stuck" bug is frustrating. My method to resolve it is as follows: Launch "Activity Monitor", click on the "CPU" tab at the top, look for the "bird" process in the "Process Name" column, select it, and stop it ("kill it") by clicking on the "Stop" icon on the tool bar (the one with an "X" inside a circle). You will noticed that the process id number listed in the "PID" column changes after a couple of seconds, which indicates that the "bird" process has been restarted by the system (The "bird" process is responsible of the iCloud background file synchronization). Most of the times this solution will work. If stopping the "bird" process did not solve the iCloud synching, then perform a full shutdown/restart of your computer. This is the ultimate fix that has always worked for me thus far.