This can usually be caused by two things, either:
1. A slow or sporadic connection to the internet. Check your internet settings, or with your Internet Service Provider if you suspect this might be the case.
2. Your upload file is very large, and your browser believes that the connection has terminated while uploading. If this seems to be the case, you should contact firstname.lastname@example.org and we will provide you with details on uploading via FTP.