Multipart.new

You're seeing just the function new, go back to Multipart module for more information.
Link to this function

new(boundary \\ generate_boundary())

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Specs

new(String.t()) :: t()

Create a new Multipart request.

Pass in the boundary as the first argument to set it explicitly, otherwise it will default to a random 16 character alphanumeric string padded by == on either side.