问题：
--jSRKAk3FJXTrWcxiPofHkPefUfGXvS
Content-Disposition: form-data; name="fileName"; filename="fileName.pdf"
Content-Type: application/octet-stream
--wGvKUCX0qMYy2I_hutbHcUsaZubpm53D
Content-Disposition: form-data; name="fileName"; filename="fileName.pdf"
--wGvKUCX0qMYy2I_hutbHcUsaZubpm53D--
--jSRKAk3FJXTrWcxiPofHkPefUfGXvS--

结论：使用MultipartEntityBuilder构建时，
会给Request中的Head增加 Content-Disposition contentType
今后多文件上传时可能会用到，需要注意

MultipartEntityBuilder build2 = MultipartEntityBuilder.create();
build2.addBinaryBody(null, file);

解决：单文件上传时，使用EntityBuilder
不会给Request中的Head增加 Content-Disposition
它设置了contentType contentEncoding

EntityBuilder builder = EntityBuilder.create();
HttpEntity httpEntity = builder.setFile(file).build();