What type of attack involves using every possible combination of letters, numbers, and characters to create candidate digests that are then matched against those in a stolen digest file?

The answer is Brute Force.  The attack that involves using every possible combination of letters, numbers, and characters to create candidate digests that are then matched against those in a stolen digest file is Brute Force. 
It is also known as brute force cracking.  It is a trial and error method used to decode passwords or Data Encryption Standard (DES) keys, through comprehensive effort (brute force).