The dyn. prefix means that the system does not know what bz2 is, so it creates a dynamic UTI. The bz2 UTI is defined by Apple, so it's probably really a problem with your system. Try cleaning the LaunchServices database with this command:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user

Please understand that after this Mac OS X will ask you again for all file types whether you really want to open them with whatever application you have installed to open the type (e.g., Word for doc, ...)