Multipart.Part.stream_body

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

stream_body(stream, headers \\ [])

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Specs

stream_body(Enum.t(), headers()) :: t()

Builds a Part with a Stream body.

Because the length of the Stream cannot be known up front it doesn't define the content_length. This will cause Multipart.content_length/1 to error unless you set the content_length manually in the struct.