In order to store an attachment in RavenDB you need to create a document. Then you can attach an attachment to the document using the session.advanced().attachments().store method.
Attachments, just like documents, are a part of the session and will be only saved on the Server when DocumentSession.saveChanges is executed (you can read more about saving changes in session here).
Attachments can be stored using one of the following session.advanced().attachments().store methods:
{CODE:java StoreSyntax@ClientApi\Session\Attachments\Attachments.java /}
{CODE:java StoreAttachment@ClientApi\Session\Attachments\Attachments.java /}