Make sure to have Mac target support installed to reproduce this issue.

Reproduction steps:
1. Open the attached project
2. Sselect the tools menu item "Build" -> "Build this"
3. Notice the error
4. Now open the script "Build.cs" and add ".app" in the end of "Hello world" (like "Hello world.app")
5. repeat step 2 and notice that error is no longer present

Actual result: "Opening file failed" error is thrown if path is without ".app" ending
Expected result: BuildPipeline.BuildPlayer should not fail to build if path does not have ".app" ending

Workaround: Adding ".app" at the end of the path fixes the problem

Reproduced on 5.4.0b16, 5.3.4p2, 5.2.4p1
Could not reproduce on 5.1.4p1