Zeptomail API with invoice attachment

I am unbale to figureout the API request (PHP) with attachment of order invoice pdf file. My code sample below -

But getting error as follows -

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

Kindly help me to fix it.

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