Everytime I click on a file in the Explorer in Visual Studio Code 1.17.2 I would like it to open it in a new tab and not replace what was it the selected tab with the clicked file's guts.

Unfortunately the undesired behavior is the default and Visual Studio has been "helping me" with something called Preview Mode as described here. You get rid of this terrible feature, go to File > Preferences > Settings ...and then splice in:

"workbench.editor.enablePreview": false

 
 

You probably need to change the dropdown at the upper right from USER SETTINGS to WORKPLACE SETTINGS too.

 
 

Addendum 11/30/2018: Also, pick "Open settings.json" from the menu that appears when you click the icon that looks like three dots in a horizontal row at the upper right in Version 1.29.1 of Visual Studio Code.

 
 

Addendum 1/20/2018: In version 1.41.1 of Visual Studio Code it says: Open Settings (JSON)