Similar to :mode and :interpreter, you can also use
:magic and :magic-fallback to cause certain function to
be run if the beginning of a file matches a given regular expression,
as if these regular expressions were added to magic-mode-alist
and magic-fallback-mode-alist (see Auto Major Mode in GNU Emacs Lisp Reference Manual). The difference between
:magic and :magic-fallback, is that the latter has a
lower priority than :mode.
Here is an example:
(use-package pdf-tools
:magic ("%PDF" . pdf-view-mode)
:config
(pdf-tools-install :no-query))
This registers an autoloaded command for pdf-view-mode, defers
loading of pdf-tools, and runs pdf-view-mode if the
beginning of a buffer matches the string "%PDF".