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/logical/media_file/image.rb

@@ -11,6 +11,16 @@ class MediaFile::Image < MediaFile
     [0, 0]
   end
 
+  def is_supported?
+    case file_ext
+    when :avif
+      # XXX Mirrored AVIFs should be unsupported too, but we currently can't detect the mirrored flag using exiftool or ffprobe.
+      !metadata.is_rotated? && !metadata.is_cropped? && !metadata.is_grid_image? && !metadata.is_animated_avif?
+    else
+      true
+    end
+  end
+
   def is_corrupt?
     image.stats
     false
@@ -28,6 +38,8 @@ class MediaFile::Image < MediaFile
       image.get("n-pages")
     elsif file_ext == :png
       metadata.fetch("PNG:AnimationFrames", 1)
+    elsif file_ext == :avif
+      video.frame_count
     else
       nil
     end
@@ -112,6 +124,10 @@ class MediaFile::Image < MediaFile
     file_ext == :png && is_animated?
   end
 
+  def is_animated_avif?
+    file_ext == :avif && is_animated?
+  end
+
   # Return true if the image has an embedded ICC color profile.
   def has_embedded_profile?
     image.icc_import(embedded: true)