Firstly - what is a .bm2 file - and how can I convert/open it on OS-X?
This appears to be a type of bitmap file (from looking at it) but a search of the web does not yield terribly useful information. I know the content of this file is several images with comments (at least).
file yourfilename.bm2
? (Run from the terminal). See here for more information on the command.