preamble-postamble-from-file.el

(defun blog/file-get-contents (file-path)
  "Returns file contents as string."
  (with-temp-buffer
	(insert-file-contents file-path)
	(buffer-string)))

(setq org-publish-project-alist
	`(
		; ...
		("blog-posts"
			; ...
		 	:html-preamble
			,(blog/file-get-contents
			(expand-file-name "header.html" blog/root-path))
		 
			:html-postamble
			,(blog/file-get-contents
			(expand-file-name "footer.html" blog/root-path)))))