How to set bit depth when saving files?

I am trying to save a bitmap with a specific bit depth. However, regardless of the type of an image, when I save it as a bitmap, it ends up with bit depth 24. I cannot find anything in the documentation when I search for "bit depth". How can I accomplish this?

A minimal working example is as follows:

using Images, FileIO
#dir = "/path/to/dir"
im_large = n0f16.(Gray.(rand(100,100)))
save(dir * "large.bmp",im_large)
im_small = n0f8.(Gray.(rand(100,100)))
save(dir * "small.bmp",im_small)

Both saved bmp files end up having bit depth 24 on my system (Windows 10).

Edit: It seems that this issue has to do with the number of channels. Even though my images above are supposedly greyscale, the images are saved with three color channels (all having equal values). An 8-bit bmp should have just a single channel.

Start by tracing the save(..., ".bmp") path used in the minimal Julia example and checking the existing documentation for image types and channels. Document how BMP bit depth is controlled, or clearly state the supported behavior and limitation; done means the example's result and the answer to the question are explained in the documentation.