In case you...
...are still on in the window from where you uploaded the files you will see the list of your confirmed uploads. Here is also a "Delete" button. Using the button you can remove individual files again.
Even though the file will be removed from the cloud storage, the owner of the CloudWok will see the file's descition in the list of files that were confirmed to be uploaded.
...have closed your upload window already, there is no way for us to differnetiate between your uploads and those of others. So you may not delete files no more.