I'm attempting to send an email with a large attachment (or attachments) using SingleEmailMessage. A EmailFileAttachment[] List is created to contain the attachments which are within the 10mb limit for all attachments but when the attachment(s) exceed 2mb in size the Apex heap size limit is exceeded. There's obviously something I'm missing here. With this in mind, how are files, of up to 10mb collectively, attached to an email using the setFileAttachments method without exceeding the Apex heap size limit?

There's no problem uploading a file of up to 5mb in size to the Attachments object for example, that doesn't cause a Apex heap size error. So I assume blobs don't count against heap size. But that doesn't seem to be true for a collection of objects that contain a blob. Surely the blob is only held by reference?

The code below doesn't enforce the 10mb attachments limit but assume it's not exceeded. When fileAttachments exceeds 2mb the 'System.Exception: Apex heap size too large' occurs.

// 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 })