Content-type: application/x-shockwave-flash
However, the browser is responsible for recognizing the content, since you can't put a flash object into IMG tag. The very fact that you're embedding with <embed> and <object> tags tells the browser that they're dealing with a flash object.
So the code for presenting the banner on the screen needs to know in advance which object it fetches, and image or flash, and along with that inserts the ID for your script that returns the object itself from the database.
Although I suggest leaving the objects and images outside the database. Have their names like img1234.jpg where 1234 is the ID of the banner in the database (with all the metadata).