ap/transmogrifier: fix attachment MIME extraction

If an attachment with a full Link object as an "url" (i.e. a map)
but no "mediaType" attribute in the Link made it to this point
processing crashed since MIME.extensions/1 requires binary input.

Additionally the is_bitstring/1 checks in the other branches were too
lax; bitstrings may contain any number of bits not just full bytes.

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -290,6 +290,10 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     |> Map.drop(["conversation"])
   end
 
+  defp is_valid_mime(mimestr) do
+    is_binary(mimestr) && MIME.extensions(mimestr) != []
+  end
+
   def fix_attachments(%{"attachment" => attachment} = object) when is_list(attachment) do
     attachments =
       Enum.map(attachment, fn data ->
@@ -302,13 +306,13 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
 
         media_type =
           cond do
-            is_map(url) && MIME.extensions(url["mediaType"]) != [] ->
+            is_map(url) && is_valid_mime(url["mediaType"]) ->
               url["mediaType"]
 
-            is_bitstring(data["mediaType"]) && MIME.extensions(data["mediaType"]) != [] ->
+            is_valid_mime(data["mediaType"]) ->
               data["mediaType"]
 
-            is_bitstring(data["mimeType"]) && MIME.extensions(data["mimeType"]) != [] ->
+            is_valid_mime(data["mimeType"]) ->
               data["mimeType"]
 
             true ->

test/pleroma/web/activity_pub/transmogrifier_test.exs

@@ -831,6 +831,109 @@ defmodule Pleroma.Web.ActivityPub.TransmogrifierTest do
 
       assert Transmogrifier.fix_attachments(object) == expected
     end
+
+    test "can deal with non-array attachment" do
+      object = %{
+        "attachment" => %{
+          "type" => "Document",
+          "name" => "Hello world",
+          "url" => "https://media.example.tld/1.jpg",
+          "mediaType" => "image/jpeg"
+        }
+      }
+
+      expected = %{
+        "attachment" => [
+          %{
+            "type" => "Document",
+            "name" => "Hello world",
+            "url" => [
+              %{
+                "type" => "Link",
+                "mediaType" => "image/jpeg",
+                "href" => "https://media.example.tld/1.jpg"
+              }
+            ],
+            "mediaType" => "image/jpeg"
+          }
+        ]
+      }
+
+      assert Transmogrifier.fix_attachments(object) == expected
+    end
+
+    test "can deal with missing MIME type" do
+      object_flat = %{
+        "attachment" => [
+          %{
+            "type" => "Document",
+            "name" => "Hello world",
+            "url" => "https://media.example.tld/1.jpg"
+          }
+        ]
+      }
+
+      object_url_map = %{
+        "attachment" => [
+          %{
+            "type" => "Document",
+            "name" => "Hello world",
+            "url" => %{
+              "type" => "Link",
+              "href" => "https://media.example.tld/1.jpg"
+            }
+          }
+        ]
+      }
+
+      expected = %{
+        "attachment" => [
+          %{
+            "type" => "Document",
+            "name" => "Hello world",
+            "url" => [
+              %{
+                "type" => "Link",
+                "href" => "https://media.example.tld/1.jpg"
+              }
+            ]
+          }
+        ]
+      }
+
+      assert Transmogrifier.fix_attachments(object_flat) == expected
+      assert Transmogrifier.fix_attachments(object_url_map) == expected
+    end
+
+    test "rejects invalid MIME type" do
+      object = %{
+        "attachment" => [
+          %{
+            "type" => "Document",
+            "name" => "Hello world",
+            "url" => "https://media.example.tld/1.jpg",
+            "mediaType" => "apzlicatonne/quark"
+          }
+        ]
+      }
+
+      expected = %{
+        "attachment" => [
+          %{
+            "type" => "Document",
+            "name" => "Hello world",
+            "url" => [
+              %{
+                "type" => "Link",
+                "href" => "https://media.example.tld/1.jpg"
+              }
+            ]
+          }
+        ]
+      }
+
+      assert Transmogrifier.fix_attachments(object) == expected
+    end
   end
 
   describe "prepare_object/1" do