IMA is a format for raw floppy-disk dumps. Many different disk formats can be expressed in this manner, but it is most commonly used for 1.44 MB PC format 3.5" floppy disks.
This is a raw, unformatted set of binary data corresponding to what is on the disk, with no headers or metadata.
This format is supported by Winimage, among other software.
.ima is the usual extension, but .img is sometimes found (though this is ambiguous since it is used for other things).