What is a .bmp file?

You found a BMP file, probably from a legacy Windows application, an old scanner, or a screenshot tool that hasn't been updated since 2003. BMP stores every pixel as raw, uncompressed colour data — no clever algorithms, no quality tradeoffs. A 12-megapixel photo saved as BMP weighs roughly 36 MB. The same image as JPG is under 5 MB.

Microsoft introduced BMP with Windows 1.0 in 1985, and the format has barely evolved since. It did what it needed to do in an era when compression was expensive and storage was measured in kilobytes-per-dollar rather than terabytes. Today, PNG provides the same lossless quality with dramatically smaller file sizes, making BMP functionally obsolete for every use case.

Every image viewer on every OS opens BMP — the format is too simple to be incompatible. If you have BMP files you want to keep, convert to PNG with fwip for lossless quality at a fraction of the size, or to JPG if you're dealing with photographs. There is genuinely no reason to create new BMP files in the current century.