Image

What is a .bmp file?

A .bmp file is an OS/2 Bitmap (version 2.0). This is an image format — a picture, drawing or graphic. Confusingly, this extension has been used by 7 different formats over the years — they're all listed below.

This extension has been used by more than one format

7 different formats have claimed .bmp over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .bmp file

GIMP (free) and IrfanView open a very wide range of image formats, including obsolete ones.
XnView MP is the best bet for unusual or legacy image types.
Your operating system's built-in viewer may work for mainstream formats.

The registry records this extension against OS/2 Bitmap, Windows Bitmap, Windows Bitmap and others, so software from that family is the most likely match.

How to convert a .bmp file

Open it in GIMP or XnView and export as .png (lossless) or .jpg (smaller).

MIME type

When a .bmp file travels over the web, servers label it with this MIME type:

image/bmp

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .bmp.