This issue has caught me out several times – but if you’re using Excel on the SharePoint server – which is common during development – you need to turn off the protected views in the ‘Trust Center’ settings of Excel:

I’ve been set straight on this issue at least twice thanks to this post by JOPX. Unless those protected views are turned off then if you download the file you will only be told the file is corrupt – even if you can download the file and open it without any problem.