Steps to reproduce:
1. Download "899017 repro.zip"
2. Move "test.php" file in project's root folder to an environment that supports PHP
3. Open "test" scene in Unity
4. Change uri field on "Test" GameObject to the "test.php" path. (http://localhost/test.php etc.)
5. Enter play mode
6. Observe "Response: DONE!" log in console
7. Observe the contents of "request.log" file (generated in the same folder as test.php)

Expected result: request.log prints out an array of information from UnityWebRequest's postData
Actual result: Array is empty

Reproduced with: 5.5.0p1, 5.5.3p1, 5.6.0f3, 2017.1.0b1
Did not reproduce with: 5.5.0f3

Regression introduced in 5.5.0p1

Note: May be related to case 826626 as the fix for another IMultipartFormSection problem was fixed for 5.5.0p1