Insert a string before the extension in a filename

Question

How can I insert a string before the extension in an image filename? For example, I need to convert this:

../Course/Assess/Responsive_Course_1_1.png

Answer 1

for files without extension and files includes extension. thanks @paul !

filename = filename.replace(/^([^.]+)$|(\.[^.]+)$/i, '$1' + "-thumb" + '$2');