Add debug UI for multi-image sends.

Signal/src/ViewControllers/ConversationView/ConversationViewItem.m

@@ -481,6 +481,7 @@ NSString *NSStringForOWSMessageCellType(OWSMessageCellType cellType)
     if (message.attachmentIds.count == 0) {
         return nil;
     }
+    // TODO: Support multi-image messages.
     NSString *_Nullable attachmentId = message.attachmentIds.firstObject;
     if (attachmentId.length == 0) {
         return nil;

Signal/src/ViewControllers/MessageDetailViewController.swift

@@ -398,6 +398,7 @@ class MessageDetailViewController: OWSViewController, MediaGalleryDataSourceDele
     }
 
     private func fetchAttachment(transaction: YapDatabaseReadTransaction) -> TSAttachment? {
+        // TODO: Support multi-image messages.
         guard let attachmentId = message.attachmentIds.firstObject as? String else {
             return nil
         }

SignalMessaging/ViewControllers/SharingThreadPickerViewController.m

@@ -579,6 +579,7 @@ typedef void (^SendMessageBlock)(SendCompletionBlock completion);
         return;
     }
 
+    // TODO: Support multi-image messages.
     NSString *_Nullable attachmentRecordId = self.outgoingMessage.attachmentIds.firstObject;
     if (!attachmentRecordId) {
         OWSLogDebug(@"Ignoring upload progress until outgoing message has an attachment record id");

SignalServiceKit/src/Messages/OWSMessageSender.m

@@ -376,11 +376,10 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
                                                      success:successHandler
                                                      failure:failureHandler];
 
-        // TODO de-dupe attachment enque logic.
-        if (message.hasAttachments) {
+        // TODO: de-dupe attachment enqueue logic.
+        for (NSString *attachmentId in message.attachmentIds) {
             OWSUploadOperation *uploadAttachmentOperation =
-                [[OWSUploadOperation alloc] initWithAttachmentId:message.attachmentIds.firstObject
-                                                    dbConnection:self.dbConnection];
+                [[OWSUploadOperation alloc] initWithAttachmentId:attachmentId dbConnection:self.dbConnection];
             [sendMessageOperation addDependency:uploadAttachmentOperation];
             [sendingQueue addOperation:uploadAttachmentOperation];
         }

SignalShareExtension/ShareViewController.swift

@@ -609,6 +609,7 @@ public class ShareViewController: UIViewController, ShareViewDelegate, SAEFailed
         }
 
         // Prefer a URL provider if available
+        // TODO: Support multi-image messages.
         if let preferredAttachment = attachments.first(where: { (attachment: Any) -> Bool in
             guard let itemProvider = attachment as? NSItemProvider else {
                 return false