feat: add file types whitelist for image processing (#885)

Add `allowedTypes` and `resizableTypes` to `imageProcessing` configuration

Prior to this commit, SVG images were not processed by `render-image.html` because SVG does not have a physical dimension like JPEG. This logic was done using a conditional.

I have now realised that Hugo can be very slow when resizing `gif` images. So I created this whitelist mechanism:

- `allowedTypes`: image types with width and height attributes
- `resizableTypes`: image types that can be resized

Here's a list of media types: bmp, gif, jpeg, png, svg+xml, tiff, webp

https://gohugo.io/templates/output-formats/#media-types

config/_default/params.yaml

@@ -11,7 +11,7 @@ featuredImageField: image
 rssFullContent: true
 
 # Path to favicon, relative to assets folder
-favicon: 
+favicon:
 
 footer:
   # Year when site was first published
@@ -146,6 +146,15 @@ colorScheme:
   default: auto
 
 imageProcessing:
+  allowedTypes:
+    - jpeg
+    - png
+    - gif
+    - webp
+  resizableTypes:
+    - jpeg
+    - png
+    - webp
   cover:
     enabled: true
   content: