fix S3 links encoding in Mediaproxy

lib/pleroma/web/media_proxy/media_proxy.ex

@@ -19,11 +19,16 @@ defmodule Pleroma.Web.MediaProxy do
     else
       secret = Application.get_env(:pleroma, Pleroma.Web.Endpoint)[:secret_key_base]
 
+      # Must preserve `%2F` for compatibility with S3 (https://git.pleroma.social/pleroma/pleroma/issues/580)
+      replacement = get_replacement(url, ":2F:")
+
       # The URL is url-decoded and encoded again to ensure it is correctly encoded and not twice.
       base64 =
         url
+        |> String.replace("%2F", replacement)
         |> URI.decode()
         |> URI.encode()
+        |> String.replace(replacement, "%2F")
         |> Base.url_encode64(@base64_opts)
 
       sig = :crypto.hmac(:sha, secret, base64)
@@ -60,4 +65,12 @@ defmodule Pleroma.Web.MediaProxy do
     |> Enum.filter(fn value -> value end)
     |> Path.join()
   end
+
+  defp get_replacement(url, replacement) do
+    if String.contains?(url, replacement) do
+      get_replacement(url, replacement <> replacement)
+    else
+      replacement
+    end
+  end
 end