ABM files are album files that will contain references to other image files. Further, ABM files may also store thumbnails of the referenced images that it uses as a preview to prevent having to load the full image unnecessarily.

Although there are many applications which may produce ABM files, the formatting of the content of these files was never standardized; this makes it important to ensure that the program you're attempting to load the ABM file with is the program that was used to create it.