How to prevent automatically opening of downloaded zip-files?

I have a problem to install an extension in Joomla. Joomla asks for a Zip-file. When I download the zip-file from the website, the zip-file is shown in Finder as a folder, i.e. an opened zip-file. Joomla does not see a folder, because it looks for a zip-file. I have a terrible solution, but I am sure others will know a better one. I used my old Windows computer, downloaded the zip-file that in Windows shows the extension .zip. Copy the file to Finder and now it shows in Finder the file as a zip-file. So, I suppose that my OS X (now Yosemite, but it happened already under Maverics) or my Safari changes a zip-file in an opened form. I could not find a way to change this to the state that Safari, Finder or OS X is not automatically opening zip-files.