When uploading a Disavow file, you should ensure your file possesses these two main factors (for successful upload):
Your file should be NO more than 2MB. If it is, reduce it by removing some of the backlinks contained in it to fall within desired size.
Please follow the format indicated by us when you are setting up "Disavow Links" in Site Explorer
Click on "Disavow Links" on your Dashboard:
You can either add links in manually or by uploading a file. By clicking "Manually Add Links", the format needed will be displayed:
See format in image below: