Zeptomail API (PHP) with invoice pdf attachment

I am unable to properly make the syntax of API request via PHP to sent order invoice (PDF) in attachment.

Below is my sample code API request -

$file = '169242759486.pdf';
$attFile = 'https://www.example.com/secureInvoice/' . $file;
$base64_attFile = chunk_split(base64_encode(file_get_contents($attFile)));
"from": { "address": "sender@email.com", "name": "Sender Name"},
"to": [{"email_address": {"address": "customer@email.com","name": "Customer Name"}}],
"reply_to": {"address": "user@email.com", "name": "User Name"},
"subject":"Test Email",
"attachments": { "content": "'.$base64_attFile.' ", "mime_type": "application/pdf", "name": "myinvoice.pdf"},
"htmlbody":"<div><b> Test email sent successfully. </b></div>",
}'

I am getting the error response as -

{"error":{"code":"TM_3301","details":[{"code":"SM_101","message":"Invalid value format","target":"attachments"}],"message":"Bad Syntax"}}

Anybody kindly help me to fix it.

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