Fallback method for calculating MD5 hashes for ServiceNow attachments. Requires HI ticket to access to the calculateMD5CheckSum() method of the GlideChecksum class from ServiceNow.

calculateAttachmentMD5-fallback.js

var grAttachment = new GlideRecord('sys_attachment'); //the table where attachment metadata is stored
grAttachment.get('0003ea666f015600623008efae3ee4f7'); //sys_id of a record in the sys_attachment table
var md5HashSum = calculateMD5Hash(grAttachment);

function calculateMD5Hash(attachmentGR) {
	var attachmentInputStream = GlideSysAttachmentInputStream(attachmentGR.sys_id + '');
	var chksum = (new GlideChecksum()).calculateMD5CheckSum(attachmentInputStream);
	gs.print(chksum);
}