Fix incorrect reaction notification copy for various attachment types.

Fixes #10141. Thanks to @Sgn-32 for the initial PR.

app/src/main/java/org/thoughtcrime/securesms/mms/SlideDeck.java

@@ -24,6 +24,7 @@ import com.annimon.stream.Stream;
 
 import org.thoughtcrime.securesms.attachments.Attachment;
 import org.thoughtcrime.securesms.util.MediaUtil;
+import org.thoughtcrime.securesms.util.Util;
 import org.whispersystems.libsignal.util.guava.Optional;
 
 import java.util.LinkedList;
@@ -148,4 +149,12 @@ public class SlideDeck {
 
     return null;
   }
+
+  public @Nullable String getFirstSlideContentType() {
+    if (Util.hasItems(slides)) {
+      return slides.get(0).getContentType();
+    }
+
+    return null;
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/notifications/DefaultMessageNotifier.java

@@ -650,6 +650,10 @@ public class DefaultMessageNotifier implements MessageNotifier {
   private static String getMessageContentType(@NonNull MmsMessageRecord messageRecord) {
     Slide thumbnailSlide = messageRecord.getSlideDeck().getThumbnailSlide();
     if (thumbnailSlide == null) {
+      String slideContentType = messageRecord.getSlideDeck().getFirstSlideContentType();
+      if (slideContentType != null) {
+        return slideContentType;
+      }
       Log.w(TAG, "Could not distinguish view-once content type from message record, defaulting to JPEG");
       return MediaUtil.IMAGE_JPEG;
     }