Added info about the boundary

Author: Nyholm

components/multipart-stream-builder.rst

@@ -31,7 +31,7 @@ A multipart stream is a special kind of stream that is used to transfer files ov
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-In the request above you see a set of HTTP headers and a body with two streams. The body starts and ends with a "boundary" and it is also this boundary that separates the streams.
+In the request above you see a set of HTTP headers and a body with two streams. The body starts and ends with a "boundary" and it is also this boundary that separates the streams. That boundary also needs to be specified in the ``Content-Type`` header.
 
 Building a multipart stream
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