When pdfBug is true, the substitution font is used in the text layer in order
to be able to know what is the font really used thanks to the devtools.
And to be sure that fonts are loaded, the font cache isn't cleaned up when
the debugger is active.
It's been loaded as a JavaScript module for a long time, and given that the file is bundled as-is (without building) it seems reasonable to just change the file extension now.