Files cannot be sent along with Wantedly messages. If you want to send a file, please ask the company's representative for their email, and send your file to them that way.
How about making your portfolio a part of your Wantedly profile? We've in included features that make it easy to beautifully display your work. Edit from here.
Consider downloading Wantedly Chat for a message app where you can attach a variety of files by copy&paste!
If you are still having trouble, contact Wantedly Customer Relations Team from here!
Sorry for the inconvenience :(