In the sandbox can you check the directory permissions on /user/hue ?
The file should be uploaded here by default (if you are logged in as hue) as this is your home HDFS directory.
As the hue user is a HUE admin but it isn’t in the HDFS superuser (which is hdfs on the Linux system) then you are not permitted to do this. You can ssh into the sandbox and change it but you should be able to upload the file to /user/hue.
What browser are you using?