Turn off attachment previews on large files

ref: https://fizzy.37signals.com/5986089/collections/7/cards/1302

lib/rails_ext/active_storage_blob_previewable.rb

@@ -0,0 +1,18 @@
+#
+#  see https://fizzy.37signals.com/5986089/collections/7/cards/1302
+#
+#  Large previewable uploads may take longer than the "pin reads" interval. So we pick a small-ish
+#  number and turn off previews for anything larger, at least until we can come up with a permanent
+#  solution.
+#
+module ActiveStorageBlobPreviewable
+  MAX_PREVIEWABLE_SIZE = 16.megabytes # this is pretty arbitrary, feel free to adjust
+
+  def previewable?
+    super && self.byte_size <= MAX_PREVIEWABLE_SIZE
+  end
+end
+
+ActiveSupport.on_load :active_storage_blob do
+  prepend ::ActiveStorageBlobPreviewable
+end