Moved creating cached preview items to didSelectAttachmentMethod

[bmourat]

During first clicking on the attachment in the feedback list, it is not displayed properly as the condition below is not true, and we cannot set current preview index properly:

if (self.cachedPreviewItems.count > [self.cachedPreviewItems indexOfObject:attachment]) {
    [previewController setCurrentPreviewItemIndex:[self.cachedPreviewItems indexOfObject:attachment]];
}