STR:
1. Download project attached and "test.php" file
2. Put "test.php" file in an environment that supports PHP
3. Open the Unity project's test scene.
4. Change uri field on Test GameObject to the "test.php" path.
5. Enter Play Mode
6. Observe "Response: DONE!" log in console
7. Check the contents of "request.log" generated in the same folder as "test.php"

Expected:
Content-type is expected to be multipart/form-data as the method uses List<IMultipartFormSection> for an argument
Actual:
Content-type is application/x-www-form-urlencoded instead.

Reproduced on: 5.2.5f1, 5.3.6p8, 5.4.0f3, 5.4.2f2, 5.5.0f9