it's very inconvenient to give users the ability to upload content that was created outside of your game (i assume you're speaking of .unity3d files and not simple images, and unity3d bundles can't even hold script files). you'd need:

• ingame tools to create content / levels
• converting the created content into a normalized form (like xml or txt)
• upload and store it on the server
• upon users requests build the level according to the stored content

depending on how complex your content / level is, the more challenging the normalization gets.

I think you might be better off to create a kind of level creator within the game. But even that would be very difficult to implement.