change * default about accepts HOT 3 CLOSED

That's correct, your array says your server-side prefers, in order (1) image/webp and (2) image/jpeg. The client does not send any Accept header, in which as in the HTTP specs, it means that the user agent accepts any type as the response, so since you most prefer to send back image/webp, that is what this module sends, as the client does accept that and it's what you prefer the most to send.

from accepts.

This is not a default this module made up: it is in the specification.

https://tools.ietf.org/html/rfc7231#section-5.3.2

A request without any Accept header field implies that the user agent
will accept any media type in response.

If you want the behavior of choosing a "fallback" when the agent accepts anything, you should list your default as the first option, so it is the one chosen when the client doesn't have any particular preference.

from accepts.

Unless I am overlooking something, this doesn't make much sense.

The input of types is the types in the order of preference, e.g.

const contentType = accepts(incomingMessage)
  .type([
    'image/webp',
    'image/jpeg',
  ]) || 'image/jpeg';

Here I prefer webp, but want to fallback to jpeg if the latter is not supported.

Having jpeg first will just mean using jpeg all the time.

from accepts.