/**
* @name OrderExtension
* @description This class is provided for you to facilitate the Super Badge
**/
public class OrderExtension {
    
    public Order orderRecord {get;set;}
    public List<OrderItem> orderItemList {get;set;}
    public String selectedFamily {get;set;}
    public List<chartHelper.chartData> pieData {get;set;}
    public Decimal total {get;set;}
    public Map<Id,OrderItem> orderItemMap;
    ApexPages.StandardSetController standardSetController;
    public OrderExtension(ApexPages.StandardController standardController){
        orderRecord = (Order)standardController.getRecord();
        orderItemMap = new Map<id,OrderItem>();
        if ( orderRecord.Id != null ){
            orderRecord = queryOrderRecord(orderRecord.Id);
        }
        resetSsc();
        total = 0;
        for (OrderItem oi : orderRecord.OrderItems) {
            orderItemMap.put(oi.Product2Id, oi);
            if (oi.Quantity > 0) {
                if (null == pieData) {
                    pieData = new List<ChartHelper.ChartData>();
                }
                pieData.add(new chartHelper.ChartData(oi.Product2.Name, oi.Quantity * oi.UnitPrice));
                total += oi.UnitPrice * oi.Quantity;
            }
        }
        PopulateOrderItems();
    }
    void resetSsc() {
        String query = 'SELECT Name, Product2.Family, Product2.Name, Product2Id, UnitPrice, Product2.Quantity_Remaining__c'
            + '  FROM PricebookEntry WHERE IsActive = TRUE';
        if (selectedFamily != null && selectedFamily != Constants.SELECT_ONE) {
            query += ' AND Product2.Family = \'' + selectedFamily + '\'';
        }
        query += ' ORDER BY Name';
        standardSetController = new ApexPages.StandardSetController(Database.getQueryLocator(query));
        standardSetController.setPageSize(Constants.DEFAULT_ROWS);
    }
    //ToDo: Implement your own method to populate orderItemList
    //  that you will call after pagination and/or family selection
    void PopulateOrderItems() {
        orderItemList = new List<OrderItem>();
        for (SObject obj : standardSetController.getRecords()) {
            PricebookEntry pbe = (PricebookEntry)obj;
            
            if (orderItemMap.containsKey(pbe.Product2Id)) {
                orderItemList.add(orderItemMap.get(pbe.Product2Id));
            } else {
                orderItemList.add(new OrderItem(
                    PricebookEntryId=pbe.Id,
                    Product2Id=pbe.Product2Id,
                    UnitPrice=pbe.UnitPrice,
                    Quantity=0,
                    Product2=pbe.Product2
                ));
            }
        }
    }
    /**
* @name OnFieldChange
* @description
**/
    public void OnFieldChange(){
        //ToDo: Implement logic to store the values changed on the page
        for (OrderItem oi : orderItemList) {
            orderItemMap.put(oi.Product2Id, oi);
        }
        //      and populate pieData
        pieData = null;
        total = 0;
        for (OrderItem oi : orderItemMap.values()) {
            if (oi.Quantity > 0) {
                if (null == pieData) {
                    pieData = new List<chartHelper.ChartData>();
                }
                pieData.add(new chartHelper.ChartData(oi.Product2.Name, oi.Quantity * oi.UnitPrice));
                //      and populate total
                total += oi.UnitPrice * oi.Quantity;
            }
        }
    }
    /**
* @name SelectFamily
* @description
**/
    public void SelectFamily(){
        //ToDo: Implement logic to filter based on the selected product family
        resetSsc();
        PopulateOrderItems();
    }
    /**
* @name Save
* @description
**/
    public void Save(){
        //ToDo: Implement logic to save the Order and populated OrderItems
        System.Savepoint sp = Database.setSavepoint();
        try {
            if (null == orderRecord.Pricebook2Id) {
                orderRecord.Pricebook2Id = Constants.STANDARD_PRICEBOOK_ID;
            }
            upsert orderRecord;
            List<OrderItem> orderItemsToUpsert = new List<OrderItem>();
            List<OrderItem> orderItemsToDelete = new List<OrderItem>();
            for (OrderItem oi : orderItemList) {
                if (oi.Quantity > 0) {
                    if (null == oi.OrderId) {
                        oi.OrderId = orderRecord.Id;
                    }
                    orderItemsToUpsert.add(oi);
                } else if (oi.Id != null) {
                    orderItemsToDelete.add(oi);
                }
            }
            upsert orderItemsToUpsert;
            delete orderItemsToDelete;
        } catch (Exception e) {
            Database.rollback(sp);
            apexPages.addMessage(new ApexPages.message(ApexPages.Severity.INFO,Constants.ERROR_MESSAGE));
        }
    }
    /**
* @name First
* @description
**/
    public void First(){
        standardSetController.first();
        PopulateOrderItems();
    }
    /**
* @name Next
* @description
**/
    public void Next(){
        standardSetController.next();
        PopulateOrderItems();
    }
    
    
    /**
* @name Previous
* @description
**/
    public void Previous(){
        standardSetController.previous();
        PopulateOrderItems();
    }
    /**
* @name Last
* @description
**/
    public void Last(){
        standardSetController.last();
        PopulateOrderItems();
    }
    /**
* @name GetHasPrevious
* @description
**/
    public Boolean GetHasPrevious(){
        return standardSetController.getHasPrevious();
    }
    
    /**
* @name GetHasNext
* @description
**/
    public Boolean GetHasNext(){
        return standardSetController.getHasNext();
    }
    
    /**
* @name GetTotalPages
* @description
**/
    public Integer GetTotalPages(){
        return (Integer)Math.ceil(standardSetController.getResultSize() / (Decimal)Constants.DEFAULT_ROWS);
    }
    
    /**
* @name GetPageNumber
* @description
**/
    public Integer GetPageNumber(){
        return standardSetController.getPageNumber();
    }
    
    /**
* @name GetFamilyOptions
* @description
**/
    public List<SelectOption> GetFamilyOptions() {
        List<SelectOption> options = new List<SelectOption>{
            new SelectOption(Constants.SELECT_ONE, Constants.SELECT_ONE)
                };
                    
                    for (Schema.PicklistEntry ple : Constants.PRODUCT_FAMILY) {
                        options.add(new SelectOption(ple.getValue(), ple.getLabel()));
                    }
        return options;
    }
    
    /**
* @name QueryOrderRecord
* @description
**/
    public static Order QueryOrderRecord(Id orderId){
        return [
            SELECT Id, AccountId, EffectiveDate, Name, Status, Pricebook2Id,
            (
                SELECT Id, OrderId, Quantity, UnitPrice, PricebookEntryId, Product2Id,
                Product2.Name, Product2.Family, Product2.Quantity_Remaining__c
                FROM OrderItems
            )
            FROM Order
            WHERE Id = :orderId
        ];
    }
}

I am building an email with Messaging.SingleEmailMessage. The email should include TXT, HTML, and ICS versions and be received by an Outlook recipient as a calendar invite (not as an email with an attachment). The key is in setting the ICS attachment as an option in the same "content-type: multipart/alternative" section as the TXT and HTML versions. However, when I build the email with setPlainTextBody, setHtmlBody, and setFileAttachments, it sets the ICS attachment in its own "content-type" section.

How can I write my code to bring the .ICS file into the "Content-type: multipart/alternative" block with the TXT & HTML?

My current code:

public static void sendEventInviteICS(String bodyTXT, String bodyHTML, String subject, String fromAddress, String displayName, List<String> toAddress, List<String> ccAddress){
    //Create Message
    Messaging.SingleEmailMessage msg = new Messaging.SingleEmailMessage();
    msg.setReplyTo(fromAddress);
    msg.setSenderDisplayName(displayName);
    msg.setToAddresses(toAddress);
    msg.setCcAddresses(ccAddress);
    msg.setSubject(subject);
    //Create TXT Version
    msg.setPlainTextBody(bodyTXT);
    //Create HTML Version
    msg.setHTMLBody(bodyHTML);
    //Create ICS Attachment
    Messaging.EmailFileAttachment efa = new Messaging.EmailFileAttachment();
    efa.setFileName('Meeting Invite.ics');
    efa.setContentType('text/calendar');
    Blob b = buildICSAttachment();
    efa.setBody(b);
    msg.setFileAttachments(new Messaging.EmailFileAttachment[]{efa});
    //Send the Email
    Messaging.sendEmail(new Messaging.SingleEmailMessage[]{msg});
}

MIME result from current code:

Date: Tue, 9 Dec 2014 21:16:26 +0000
From: Isaac Lewis <ilewis@afs.net>
Reply-To: <djillumine@gmail.com>
To: "ilewis@afs.net" <ilewis@afs.net>
Subject: Sandbox: This is the subject
MIME-Version: 1.0
Content-type: multipart/mixed;
	boundary="B_3500983009_1157947"

> This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible.

--B_3500983009_1157947
Content-type: multipart/alternative;
	boundary="B_3500983009_1123381"


--B_3500983009_1123381
Content-type: text/plain;
	charset="US-ASCII"
Content-transfer-encoding: 7bit

This is the text body


--B_3500983009_1123381
Content-type: text/html;
	charset="US-ASCII"
Content-transfer-encoding: quoted-printable

<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Dutf-8">
</head>
<body>
<i>This is the html body</i>
</body>
</html>


--B_3500983009_1123381--


--B_3500983009_1157947
Content-type: text/calendar; name="Attachment Name.ics"
Content-disposition: attachment;
	filename="Attachment Name.ics"
Content-transfer-encoding: base64

QkVHSU46VkNBTEVOREFSDQpNRVRIT0Q6UkVRVUVTVA0KUFJPRElEOk1pY3Jvc29mdCBFeGNo=

--B_3500983009_1157947--

Desired MIME Result (boundaries around ICS have been moved to be part of the "Content-type: multipart/alternative" block):

Date: Tue, 9 Dec 2014 21:16:26 +0000
From: Isaac Lewis <ilewis@afs.net>
Reply-To: <djillumine@gmail.com>
To: "ilewis@afs.net" <ilewis@afs.net>
Subject: Sandbox: This is the subject
MIME-Version: 1.0
Content-type: multipart/mixed;
	boundary="B_3500983009_1157947"

> This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible.

--B_3500983009_1157947
Content-type: multipart/alternative;
	boundary="B_3500983009_1123381"


--B_3500983009_1123381
Content-type: text/plain;
	charset="US-ASCII"
Content-transfer-encoding: 7bit

This is the text body


--B_3500983009_1123381
Content-type: text/html;
	charset="US-ASCII"
Content-transfer-encoding: quoted-printable

<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Dutf-8">
</head>
<body>
<i>This is the html body</i>
</body>
</html>

--B_3500983009_1123381
Content-type: text/calendar; name="Attachment Name.ics"
Content-disposition: attachment;
	filename="Attachment Name.ics"
Content-transfer-encoding: base64

QkVHSU46VkNBTEVOREFSDQpNRVRIT0Q6UkVRVUVTVA0KUFJPRElEOk1pY3Jvc29mdCBFeGNo=

--B_3500983009_1123381--

--B_3500983009_1157947--