基本的に、データの形式は次のようになります (HTTP 標準形式に従う必要があります)。
Content-Type: multipart/form-data; boundary=--3i2ndDfv2rTHiSisAbouNdArYfORhtTPEefj3q2f
--3i2ndDfv2rTHiSisAbouNdArYfORhtTPEefj3q2f
Content-Disposition: form-data; name="foo1.jpg"; filename="foo1.jpg"
Content-Length: 5578
Content-Type: image/jpeg
<image data 1 omitted>
--3i2ndDfv2rTHiSisAbouNdArYfORhtTPEefj3q2f
Content-Disposition: form-data; name="foo2.jpg"; filename="foo2.jpg"
Content-Length: 327
Content-Type: image/jpeg
<image data 2 omitted>
--3i2ndDfv2rTHiSisAbouNdArYfORhtTPEefj3q2f--