Is
Returns the matching content type, if the incoming request’s Content-Type HTTP header field matches the MIME type specified by the type parameter.
If the request has no body, it returns false.
func (c *Ctx) Is(extension string) bool
// Content-Type: text/html; charset=utf-8
app.Get("/", func(c *fiber.Ctx) error {
c.Is("html") // true
c.Is(".html") // true
c.Is("json") // false
// ...
})