Allow mp4 uploads, convert to WebM

app/models/media_attachment.rb

@@ -1,6 +1,6 @@
 class MediaAttachment < ApplicationRecord
   IMAGE_MIME_TYPES = ['image/jpeg', 'image/png', 'image/gif'].freeze
-  VIDEO_MIME_TYPES = ['video/webm'].freeze
+  VIDEO_MIME_TYPES = ['video/webm', 'video/mp4'].freeze
 
   belongs_to :account, inverse_of: :media_attachments
   belongs_to :status,  inverse_of: :media_attachments
@@ -43,6 +43,11 @@ private
       }
     else
       {
+        original: {
+          convert_options: {},
+          format: 'webm'
+        },
+
         small: {
           convert_options: {
             output: {