Fix #431 - convert gif to webm during upload. Web UI treats them like it did

before. In the API, attachments now can be either image, video or gifv. Gifv
is to be treated like images in terms of behaviour, but are videos by file
type.

lib/paperclip/gif_transcoder.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Paperclip
+  # This transcoder is only to be used for the MediaAttachment model
+  # to convert animated gifs to webm
+  class GifTranscoder < Paperclip::Processor
+    def make
+      num_frames = identify('-format %n :file', file: file.path).to_i
+
+      return file unless options[:style] == :original && num_frames > 1
+
+      final_file = Paperclip::Transcoder.make(file, options, attachment)
+
+      attachment.instance.file_file_name    = 'media.webm'
+      attachment.instance.file_content_type = 'video/webm'
+      attachment.instance.type              = MediaAttachment.types[:gifv]
+
+      final_file
+    end
+  end
+end