Error uploading Dawn template modified

I’m trying to upload a modified version of the dawn theme, I’ve done it a few times before and so far I haven’t had any problems.
I have generated a file with gulp using the npm run zip command. The problem I have is that in the Ghost editor when I try to upload the file I get the error “The file type you uploaded is not supported”.

I tried to analyse the output zip file with “gscan” command and it didn’t prompt any error.
Looking at the browser debug console the error is as follows;

It seems to be sending that the file type is json when it should be zip. Is this the case? Does anyone else have this problem?