Hi Josef ,
I'm unable to reproduce this issue .All the Ads type files upload to studio without any issue.
Is this Ad created as banner or expandable type in GWD ? Did you edit the file in code view ?
Can you also check if you are using the latest GWD version 1.3.2.0521 .
If possible ,could you please send us your source .zip file @ gwd-s...@google.com
so we can debug this further .