extension method returning undefined for `text/plain`

console.log(
    "text/plain",
    extension("text/plain"),
    extensionsByType("text/plain"),
    parseMediaType("text/plain"),
    [...extensions.entries()]
);

console.log(
    "text/plain",
    extension("text/plain"),
    extensionsByType("text/plain"),
    parseMediaType("text/plain"),
    [...extensions.entries()]
);

text/plain undefined undefined [ "text/plain", undefined ] []

text/plain undefined undefined [ "text/plain", undefined ] []