Using Linux when I download the project file it is of type "tar.gz"
(I don't know why but it seems this recently change.. before was just zip file).
But it seems that the MIME type of the file is not correctly set by Cook.
Please set the mime data for tar.gz to

"application/x-gzip"
or even
application/x-tar
will work...

Strange.

application/x-gzip is used for the moment.

Can you tell me wich MIME is returned ?
Maybe the both are required in the component ?

Oh yeah, of course. I must try but really now I am busy, I try to have a look at least to fix it in V2.0

TODO :
In your model, where the attempted MIME/EXT are setted, try to add : application/x-tar

I am sorry but I did not catch you... what do you mean by "in your model..."
Do you mean it's up to the jcook user to define this mime type ?
There is no place (as far as I know) to define this...

When you download generated component you mean...

Ok, I check this.

Is it better now ?

That's all OK.