.dat usually means that the file is a data file. Many different applications the .dat extension for many different types of content. Which application(s) created your .dat file?
It was a programme (?) which converted from a Canon analog tape to MPEGAV CD and file is AVSEQ.dat . Quick time plays it and I am sure Handbrake or Toaster on my iMac would do the job, but presently I am elsewhere hence this query !
Need to down load a software from web, which does not cost the moon and will do the job