Is this a Raw file? One would think so -- you should understand that an "image viewer" will show the jpeg that is "embedded" in a Raw file, so if you set your camera to the Monochrome Picture Style then that's what you will see.
However, the Raw data will be "untouched" by your Picture Style setting. The Canon Raw processing software Digital Photo Professional (DPP) "reads" and applies those in-camera settings to give you an initial Raw preview that will show that.
But other Raw processors, including Adobe Camera Raw which comes with Photoshop CSx, does not read and apply those settings, but rather applies a "default" process which renders an RGB/color image, and then has tools for Grayscale conversion and manipulation, and also offers numerous Grayscale/B&W presets that you can use -- just realize that the Raw file data is the same either way.