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SMIL specifies how MMS media files are laid out in MMS. Telnyx generates SMIL automatically based on media_urls and text fields. Advanced users can add a smil_template parameter to the MMS message request. This template will then be used to generate a customized SMIL. Example of SMIL template: 
<smil>
	<head>
		<region id="Text" top="70%" left="0%" height="30%" width="100%" fit="scroll"/>
		<region id="Image" top="0%" left="0%" height="70%" width="100%" fit="meet"/>
	</head>
	<body>
		<par dur="10s">
			<text src="{{ text }}" region="Text" />
			<img src="{{ 0 }}" region="Image" />
		</par>
	</body>
</smil>
Note that the template has parameters enclosed in double braces {{ }}. During the construction of SMIL these parameters will be automatically replaced with locations of text and media contents. In case of media, {{ 0 }} will be replaced with the location of the first URL in the media_urls list, {{ 1 }} will be replaced with the location of the second URL in the media_urls list, and so on. {{ text }} will be replaced with the location of the text content. The SMIL template must be set in smil_template parameter of the MMS request. It must be properly JSON-escaped: 
{
	"from": "+13125551234",
	"to": "+18655551234",
	"text": "Hello from Telnyx",
	"media_urls": ["https://example.com/media.jpg"],
	"smil_template": "<smil>\n\t<head>\n\t\t<region id=\"Text\" top=\"70%\"..."
}
Please note that many modern handsets (most notably iPhones) ignore SMIL and use their own layout algorithm.