FtpFileHelper.cs

    public static class ZipHelper
    {
        const string ZipExt = ".zip";

        public class FileContent : IDisposable
        {
            public readonly string FileName;
            public readonly string NameInArchive;
            public readonly Stream Content;

            public FileContent(string fileName, Stream content, string nameInArchive)
            {
                FileName = fileName;
                NameInArchive = nameInArchive;
                Content = content;
            }

            public bool IsInArchive { get { return NameInArchive != null; } }
            public string Name { get { return NameInArchive ?? FileName; } }
            public string Extension { get { return Path.GetExtension(Name); } }

            public void Dispose()
            {
                Content.Dispose();
            }
        }

        public static IEnumerable<FileContent> GetContents(string fileName, Stream stream)
        {
            var ext = Path.GetExtension(fileName).ToLowerInvariant();
            if (ext == ZipExt)
                using (var zip = new ZipArchive(stream))
                    foreach (var entry in zip.Entries)
                        yield return new FileContent(fileName, entry.Open(), entry.Name);
            else
                yield return new FileContent(fileName, stream, null);
        }
    }

FtpFileHelperUse1.cs

        public override ExternalLoaderResult<FileItem> ProccessFile(FileItem item, Stream stream)
        {
            var result = new ExternalLoaderResult<FileItem>(item);
            foreach (var content in ZipHelper.GetContents(item.Path, stream))
                if (content.Extension == XmlExt)
                    try
                    {
                        TransformXml(result, content.Content, content.Name);
                    }
                    catch (Exception ex)
                    {
                        result.AddError("Error in " + content.Name, ex);
                    }
                    finally
                    {
                        content.Dispose();
                    }
            return result;
        }

FtpFileHelperUse2.cs

        public IEnumerable<TExternal> GetExternalDocuments()
        {
            foreach (var fileItem in FileEnumerator.Enumerate())
                foreach (var content in ZipHelper.GetContents(fileItem.Path, FileEnumerator.GetFile(fileItem.Path)))
                    using (content)
                        yield return Deserialize(content.Content);
        }