I succeeded to make it work, by setting the post body as follow :

 data = %{--bbe873dc\r
Content-Type: application/atom+xml; charset=UTF-8

#{xml_entry}\r
--bbe873dc\r
Content-Type: #{meta[:file].content_type}
Content-Transfer-Encoding: binary

#{meta[:file].read}\r
--bbe873dc--\r\n}

I still don't understand why it wasn't working properly before ... :/
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