This is a regression from MDL-38388.
Import of .zip archives is working as expected, but import of .dat files is broken.
Notice: Undefined variable: text in C:\wamp\www\moodle_head\question\format\blackboard_six\format.php on line 81
Call Stack

1. Time Memory Function Location
   1 0.0028 743760 {main}( ) ..\import.php:0
   2 1.8617 82028992 qformat_default->importprocess( ) ..\import.php:119
   3 1.8620 82029176 qformat_blackboard_six->readdata( ) ..\format.php:298
   
   Notice: Undefined variable: text in C:\wamp\www\moodle_head\question\format\blackboard_six\format.php on line 84
   Call Stack
   # Time Memory Function Location
   1 0.0028 743760 {main}

   ( ) ..\import.php:0
   2 1.8617 82028992 qformat_default->importprocess( ) ..\import.php:119
   3 1.8620 82029176 qformat_blackboard_six->readdata( ) ..\format.php:298
   fileobj type not recognised

line 182 of \question\format\blackboard_six\format.php: call to debugging()
line 303 of \question\format.php: call to qformat_blackboard_six->readquestions()
line 119 of \question\import.php: call to qformat_default->importprocess()