fix avatar content type

Closes #5417
// FREEBIE

src/org/thoughtcrime/securesms/groups/GroupManager.java

@@ -104,7 +104,7 @@ public class GroupManager {
 
     if (avatar != null) {
       Uri avatarUri = SingleUseBlobProvider.getInstance().createUri(avatar);
-      avatarAttachment = new UriAttachment(avatarUri, ContentType.IMAGE_JPEG, AttachmentDatabase.TRANSFER_PROGRESS_DONE, avatar.length);
+      avatarAttachment = new UriAttachment(avatarUri, ContentType.IMAGE_PNG, AttachmentDatabase.TRANSFER_PROGRESS_DONE, avatar.length);
     }
 
     OutgoingGroupMediaMessage outgoingMessage = new OutgoingGroupMediaMessage(groupRecipient, groupContext, avatarAttachment, System.currentTimeMillis());