_ActionTextAttachmentUploadNode should implement importDOM for proper HTML serialization

[Xeross99]

I've noticed the following warning in the console when using the editor:

[Warning] _ActionTextAttachmentUploadNode should implement "importDOM" if using a custom "exportDOM" method to ensure HTML serialization (important for copy & paste) works as expected

Details:

• Package version: @37signals/lexxy 0.1.22-beta (installed via Yarn)
• Warning occurs during editor initialization