Fix #5065: .webp images upload support

Add ability to upload .webp images.

Animated WebP images aren't supported. This is because they aren't
supported by FFmpeg yet[1], so generating thumbnails and samples for
them would be more complicated than for other formats.

[1]: https://trac.ffmpeg.org/ticket/4907

app/logical/media_file.rb

@@ -26,7 +26,7 @@ class MediaFile
     file = Kernel.open(file, "r", binmode: true) unless file.respond_to?(:read)
 
     case file_ext(file)
-    when :jpg, :gif, :png, :avif
+    when :jpg, :gif, :png, :webp, :avif
       MediaFile::Image.new(file, **options)
     when :swf
       MediaFile::Flash.new(file, **options)
@@ -62,6 +62,10 @@ class MediaFile
     when /\A\x1a\x45\xdf\xa3/n
       :webm
 
+    # https://developers.google.com/speed/webp/docs/riff_container
+    when /\ARIFF....WEBP/
+      :webp
+
     # https://www.ftyps.com
     # isom (common) - MP4 Base Media v1 [IS0 14496-12:2003]
     # mp42 (common) - MP4 v2 [ISO 14496-14]
@@ -144,7 +148,7 @@ class MediaFile
 
   # @return [Boolean] true if the file is an image
   def is_image?
-    file_ext.in?([:jpg, :png, :gif, :avif])
+    file_ext.in?(%i[jpg png gif webp avif])
   end
 
   # @return [Boolean] true if the file is a video