uploads: add support for uploading .avif files.

Features of AVIF include:

* Lossless and lossy compression.
* High dynamic range (HDR) images
* Wide color gamut images (i.e. 10- and 12-bit color depths)
* Transparency (through alpha planes).
* Animations (with an optional cover image).
* Auxiliary image sequences, where the file contains a single primary
  image and a short secondary video, like Apple's Live Photos.
* Metadata rotation, mirroring, and cropping.

The AVIF format is still relatively new and some of these features aren't well
supported by browsers or other software:

* Animated AVIFs aren't supported by Firefox or by libvips.
* HDR images aren't supported by Firefox.
* Rotated, mirrored, and cropped AVIFs aren't supported by Firefox or Chrome.
* Image grids, where the file contains multiple images that are tiled
  together into one big image, aren't supported by Firefox.
* AVIF as a whole has only been supported for a year or two by Chrome
  and Firefox, and less than a year by Safari.

For these reasons, only basic AVIFs that don't use animation, rotation,
cropping, or image grids can be uploaded.

app/models/media_asset.rb

@@ -3,7 +3,7 @@
 class MediaAsset < ApplicationRecord
   class Error < StandardError; end
 
-  FILE_TYPES = %w[jpg png gif mp4 webm swf zip]
+  FILE_TYPES = %w[jpg png gif avif mp4 webm swf zip]
   FILE_KEY_LENGTH = 9
   VARIANTS = %i[preview 180x180 360x360 720x720 sample original]
   MAX_FILE_SIZE = Danbooru.config.max_file_size.to_i
@@ -273,6 +273,8 @@ class MediaAsset < ApplicationRecord
       def validate_media_file!(media_file, uploader)
         if !media_file.file_ext.to_s.in?(FILE_TYPES)
           raise Error, "File is not an image or video"
+        elsif !media_file.is_supported?
+          raise Error, "File type is not supported"
         elsif media_file.is_corrupt?
           raise Error, "File is corrupt"
         elsif media_file.file_size > MAX_FILE_SIZE
@@ -370,7 +372,7 @@ class MediaAsset < ApplicationRecord
 
   concerning :FileTypeMethods do
     def is_image?
-      file_ext.in?(%w[jpg png gif])
+      file_ext.in?(%w[jpg png gif avif])
     end
 
     def is_static_image?

app/models/post.rb

@@ -188,7 +188,7 @@ class Post < ApplicationRecord
     end
 
     def is_image?
-      file_ext =~ /jpg|gif|png/i
+      file_ext =~ /jpg|gif|png|avif/i
     end
 
     def is_flash?