I've just come across this post because I was wondering why my project is not opening and I am receiving an error message about the file type. I viewed the console and couldn't make sense of what the problem was. I did change the file name/type from ".c3p" to ".zip", extracted - then confirm if the file had the correct image and it was confirmed to me. I was not readily aware of the "exact" problem until I seen this post. So I closed out the spaces in the image name and made the corrections.
However, When I changed it back to .c3p, it proceeded to show me another error in the console. Fixed it again, compress, extract etc and THEN it showed me another error in the console etc. This maybe a problem for alot of the objects I have in my game.
The reality is, that like many of us developers who are so far into development- my game is HUGE! There are a multitude (perhaps) hundreds of objects in the project that could have this issue.
This really is annoying considering that I cannot use it on the current version of construct. Granted, I was able to open the project in r388.2, Though admittedly I would hope they fix this.