From what I know MIP is a method for texture map projection, not the file format.
Well from how i saved it and what Paul said, that its supposed to save a progressively smaller version of the image so that you have finer control over how the image, in this case the logos scale when you zoom in and out. In game atm, custom logos look rather jaggy at certain view distances. and this is supposed to reduce that as your able to save and edit each stage of the image scaling to remove artifacts you dont like or just generally make it look "better".
This can all be saved within a .dss file format, so while Mip itself may not be a format, it can be a part of the .dss format if you choose to use it.
Example image below
Quote from wiki "In 3D computer graphics, mipmaps (also MIP maps) are pre-calculated, optimized sequence of textures that accompany a main texture, each of which is a progressively lower resolution representation of the same image. The height and width of each image, or level, in the mipmap is a power of two smaller than the previous level. Mipmaps do not have to be square. They are intended to increase rendering speed and reduce aliasing artifacts. "