1. Load up attached project (OC link in comments, file name "940942-UploadTest.zip")
2. Build for WebGL on at least two versions (one it works with and one it does not work with)
3. Host somewhere
4. Load up the build on an internet browser
5. Press Upload Image (click twice)
6. Select any image from the hard drive
7. The image will either get loaded or not depending on which version the project was built with

OR

1. Download attached builds (2017.1b3 for one that doesn't work (171b3-build.zip), and 5.6.3p1 for one that works (563p1-build.zip))
2. Continue at #3

Expected result: images are loaded onto the cube in any build
Actual result: no image is loaded because the returned path drops slashes

Reproducible with: 2017.1.0b3, 2017.1.0b4, 2017.1.0b5, 2017.1.0f3
Not reproducible with: 5.6.2f1, 5.6.3p1, 2017.1.0a1
Regression introduced in: 2017.1.0b3

Note: the project did not build on 2017.1.0b1 and 2017.1.0b2 for some reason so there's a slight chance the regression might have been introduced there