move create-thumbnail code to utils (reduce duplicated code).

src/utils.lisp

@@ -17,7 +17,8 @@
 	   #:asciify
 	   #:slugify
 	   #:get-image-dimensions
-	   #:run-bash-command)
+	   #:run-bash-command
+	   #:create-thumbnail)
   (:documentation "Utilities that do not depend on models."))
 
 (in-package #:utils)
@@ -100,3 +101,14 @@ The `FILEPATH' must be already merged with
   (uiop:run-program command :output :string
 			    :ignore-error-status t
 			    :error-output :string))
+
+(defun create-thumbnail (file-name)
+  "Runs a Bash command to convert a file to a thumbnail  in /storage/media dir.
+The file is reduced to 512x512 pixels if bigger than that. A new file
+is then created with a 'thumbnail-' pre-fix. This process relies on
+Image Magick. So, it must be installed on the system for this function
+to operate properly."
+  (run-bash-command
+  (format nil "convert ~a -resize 512x512\\> ~a"
+	  (storage:file-exists-p "" "media" file-name)
+	  (storage:make-path "" "media" (format nil "thumbnail-~a" file-name)))))