Return a Zotero.Item from all Zotero.Attachments methods

These previously returned an itemID, but now that new saved items can be edited
without a refetch, they should just return the new item.

(Possible I missed a few spots where these are called.)

test/tests/collectionTreeViewTest.js

@@ -236,7 +236,7 @@ describe("Zotero.CollectionTreeView", function() {
 			});
 			var file = getTestDataDirectory();
 			file.append('test.png');
-			var attachmentID = yield Zotero.Attachments.importFromFile({
+			yield Zotero.Attachments.importFromFile({
 				file: file,
 				parentItemID: item.id
 			});