add thumbnail path for SVG file in build-thumbnail-path filter.

The file-type contains 'image' so the filter assumed it was a .png or .jpg and
the image wouldn't render when it was a .svg file. This additional check to the
filter makes sure the svg.png (stock image file) is used when a thumbnail is
used in a HTML template.

src/view.lisp

@@ -70,7 +70,9 @@
   (format nil "~S" (if (= 1 value) "on" "off")))
 
 (djula:def-filter :build-thumbnail-path (file)
-  (cond ((str:contains? "image" (files::file-type-of file) :ignore-case t)
+  (cond ((str:contains? "svg" (files::file-type-of file) :ignore-case t)
+	 (format nil  "/images/icons/svg.png"))
+        ((str:contains? "image" (files::file-type-of file) :ignore-case t)
 	 (format nil  "/storage/thumb/media/~a" (files::slug-of file)))	
 	((str:contains? "pdf" (files::file-type-of file) :ignore-case t)
 	 (format nil  "/images/icons/pdf.png"))