test: add test for normalizeType fallback when mime lookup fails (#6894)

Add test to verify that utils.normalizeType correctly defaults to
'application/octet-stream' when mime.lookup() returns null/undefined
for unknown file extensions. This covers the fallback behavior on
line 64 of lib/utils.js and ensures proper handling of unrecognized
MIME types.

Co-authored-by: bjohansebas <103585995+bjohansebas@users.noreply.github.com>