I'm sending an email with attachments using SingleEmailMessage. An EmailFileAttachment[] List is created to contain the attachments which are within the specified 10mb limit for all attachments but when the attachments exceed 2mb in size the Apex heap size limit is exceeded.

My question simply is this: how are files, of up to 10mb, attached to an email using the SingleEmailMessage class without getting 'System.Exception: Apex heap size too large'?

// Create EmailMessage and assign parameters
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage(); 
mail.setToAddresses(toAddresses);
mail.setCcAddresses(ccAddresses);
mail.setReplyTo('support@acme.com');
mail.setSenderDisplayName('Support');
mail.setSubject('New Case Created : ' + case.Id);
mail.setBccSender(false);
mail.setUseSignature(false);
mail.setPlainTextBody('Your Case: ' + case.Id +' has been created');
mail.setTargetObjectId(case.contactId);
mail.setWhatId(case.Id);

// Set email file attachments
List<Messaging.Emailfileattachment> fileAttachments = new List<Messaging.Emailfileattachment>();
for (Attachment a : [select Name, Body, BodyLength 
                     from Attachment
                     where ParentId = :case.Id])
{
  // Add to attachment file list
  Messaging.Emailfileattachment efa = new Messaging.Emailfileattachment();
  efa.setFileName(a.Name);
  efa.setBody(a.Body);
  fileAttachments.add(efa);
}
mail.setFileAttachments(fileAttachments);

// Send email
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail })

I came up against this issue a couple of months back, posted it, but got no response. So apologies, I'm posting again in the hope that I can finally find a solution.

Would appreciate any responses on this one, even if it's half a guess. Thanks in advance.