App Cannot be Selected as the Tool for Diffing Text on a Mac

Editor Preferences -> General -> Loading & Saving> Source Control: the "Tool for diffing text" dialog box doesn't let you choose an application, because the file extension is set to .exe (for Windows) but there is not an .app option for Mac.
(See image)

Regression: No

1. Open any UE4.15 Project on a Mac
2. Navigate to Editor Preferences -> General -> Loading & Saving> Source Control
3. Open the search window to choose a "Tool for diffing text"
4. Navigate to the Applications folder and attempt to open any app.
   RESULT: The only file extension available to select is .exe so that apps, ending in extension .app are not available for selection
   EXPECTED: The extension ".app" to be a choice in the drop down menu, thus allowing the user to select an .app as the tool for diffing text on a Mac.