As Jim mentioned, the device you are looking for is called a pad. It will drop the level as necessary, you can either get an adjustable one or simply get a 60db pad and you should be ok, but you should also consider whether your current approach is the best.
If you are playing back some electronic file in to the mic input, it would be a better bet to simply combine the audio electronically in video editing software after you have captured the video. This is a process called dubbing and is done regularly to achieve higher quality results. The reason is that it avoids something called generations of loss. Any time that you reproduce an analog signal, a portion of the signal is going to be lost. The more times you reproduce it, the worse it will get. A lot of effort goes in to minimizing the generations of loss and is a big part of why lossless digital work flows are so popular.