Currently the AppStore is bombarded with download requests, which sometimes seems to fail or result in corrupted downloads. The AppStore team is looking into it. We're sorry for the inconvenience.
EDIT: Problem should be solved now. Happy downloading!
I'm experiencing the same problem....
Had to download it again twice, then it was ok
It looks like the download was interrupted half way. These are the checksums of Mendix-7.0.2-Setup.exe:
md5: 12fa9a3670478af7ec9416ec76b6b89a
sha1: 529e569f8313d32cefc578d9cd2bdc12d2cb3d69
sha256: 717e51fd5d7ab1cd77493bc42e56a3fc3f4a3e4707d75f20cec13c97eaf9ec24