Steps to reproduce:
1. Open build settings on any platform
2. Click Build
3. As the filename for your build write in the name of a folder in the same directory
For example: if there's a folder called "Temp" try to create a "Temp.exe" build file
4. Instead of creating the file when you click "save" it will open the folder called "Temp"

Expected: If the button says "Save" I expect Unity to create a file with that name and extension, not open a folder with that name
Actual: It just opens the folder with that build name you are trying to create

Reproduced on: 2018.1.0b4, 2017.3.0p3, 2017.2.1p2, 2017.1.3f1, 5.6.5p1