Easy enough to fix: Rename the file so the extension is .zip instead of .jar...if you REALLY need a .zip file. But you don't.
Note that the OP said he is using Magic Launcher. Magic Launcher does not require mods to have a .zip extension, you can use .jar files. The important thing is that you have a clean minecraft.jar file (which you can specify), it's modloader to be first, then forge, then your mods.
To the OP: Honestly, take a few minutes to learn the new launcher, it will work well for what you need. The only problem is that it doesn't allow you to add mods graphically. With the Forge installer, modding Minecraft for Forge compatible/required mods is dead simple. DEAD. SIMPLE. You'll have your Forge profile which you can copy to create different flavors of Forge. There are instructions here in the Suggestions forum to help you get started.
To LexManos: To be honest, when I went to install Forge for 1.6.x, I grabbed the universal zip file like I always did. When I opened it to install Forge, I did not find the files I expected and spent a few minutes tracking down what happened. The new installer happened without fanfare (at least I didn't see anything posted about) so I was confused. I love the new installer, btw. Makes things wonderfully easy to work with now.