From the keyboard of Dr Jan

As you may know, the prefix mega- is abbreviated with an upper case ‘M’. So, for example, 5 megabytes can be written as 5MB. A single megabyte is either 1,000,000 or 1,048,576 bytes.
From the Wikipedia:
In computing, mega- can sometimes denote 1,048,576 (220) of information units (example: a megabyte, a megaword), but can denote 1,000,000 (106) [...]