How do I search by file extension in MacOS Finder?
OK, I finally figured it out with the following approach:


Open finder to the top-level directory you wish to search
Press ⌘ + F
From the search menu options, on the left, choose other
Click the checkbox under the In Menu column for file extension from the popup window and close the popup window by pressing OK
Select file extension from the search menu
Type your file extension in the text field (follows the word is)