iCalendar invite is not showing RSVP buttons in email clients

I'm using PHPMailer to send email. My ics content is:

BEGIN:VCALENDAR
METHOD:REQUEST
VERSION:2.0
PRODID:-//FooBar//FooBar Calendar//EN
BEGIN:VEVENT
UID:8f5a0777-bf6d-17d2-f14a-52e7feedf810
ORGANIZER;CN=Foo:foo@bar.com
DTSTART:20140130T191500Z
DTEND:20140130T194500Z
DTSTAMP:20140129T144300Z
SUMMARY:Foo Bar 11
DESCRIPTION:
END:VEVENT
END:VCALENDAR

For some reason I don't see the expected RSVP buttons in any email clients. Both Gmail and Mac Mail simply include the ics as a file attachment, I assume as a fallback. So there must be something wrong. I have not tried Outlook, but I don't see any reason why it would be different in a third client. The contents of an email being sent are below. Can you see anything that looks incorrect? Thank you.

Return-Path: REDACTED
Received: REDACTED
Date: Wed, 29 Jan 2014 09:43:18 -0500
Return-Path: REDACTED
To: REDACTED
From: REDACTED
Reply-To: REDACTED
Subject: Meeting - Foo Bar 11
Message-ID: <7004961cd9df04181047296f98260834@localhost>
X-Priority: 3
X-Mailer: PHPMailer 5.2.7 (https://github.com/PHPMailer/PHPMailer/)
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="b1_7004961cd9df04181047296f98260834"
Content-Transfer-Encoding: base64

--b1_7004961cd9df04181047296f98260834
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: base64

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


--b1_7004961cd9df04181047296f98260834
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: base64

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


--b1_7004961cd9df04181047296f98260834
Content-Type: text/calendar; method=REQUEST; charset=UTF-8
Content-Transfer-Encoding: base64

QkVHSU46VkNBTEVOREFSDQpNRVRIT0Q6UkVRVUVTVA0KVkVSU0lPTjoyLjANClBST0RJRDotLy9Gb29CYXIvL0Zvb0JhciBDYWxlbmRhci8vRU4NCkJFR0lOOlZFVkVOVA0KVUlEOjhmNWEwNzc3LWJmNmQtMTdkMi1mMTRhLTUyZTdmZWVkZjgxMA0KT1JHQU5JWkVSO0NOPUZvbzpmb29AYmFyLmNvbQ0KRFRTVEFSVDoyMDE0MDEzMFQxOTE1MDBaDQpEVEVORDoyMDE0MDEzMFQxOTQ1MDBaDQpEVFNUQU1QOjIwMTQwMTI5VDE0NDMwMFoNClNVTU1BUlk6Rm9vIEJhciAxMQ0KREVTQ1JJUFRJT046DQpFTkQ6VkVWRU5UDQpFTkQ6VkNBTEVOREFSDQo=



--b1_7004961cd9df04181047296f98260834--

P.S. Sorry about it being base64 encoded. I wanted to show a true representation of the email instead of changing it any way that may skew the results. I just use http://www.base64decode.org/ to decode the contents.

P.P.S. The base64 contents show as one string, but the email is sent with a max of 1000 characters per line. I just needed to remove some sensitive information, so I had to re-encode those parts, which removed the breaks.