The answer depends on exactly how you want to interact with the S-RAMP repository.  But first let me start with just a little bit of background.  The S-RAMP spec does not include a way to define your custom types - so they are all ad-hoc and they all live within the /s-ramp/ext model.  So unfortunately it is not possible to define some sort of schema that defines your types and bind that schema to a custom model, such as "/s-ramp/foo".

What this means is that defining your own types is simply a matter of adding artifacts to the repository with a custom type name. So instead of adding (for example) a WsdlDocument, you can add a FooDocument or a FooArtifact.  Once such an artifact has been added, you can treat it like any other artifact - adding properties, classifiers, and relationships to it.  You can obviously query for it by all of that criteria as well.

Finally, it's important to note that there are two kinds of extended artifacts - ExtendedDocument and ExtendedArtifactType.  The former has binary (e.g. file) content and the latter does not.  So if you want to create an artifact that consists *only* of meta-data (properties, classifiers, relationships) then you would use ExtendedArtifactType.  If you are storing a file as an artifact (for example uploading a JBoss 7 SAR file as a SecurityArchive artifact) then you would use ExtendedDocument.

S-RAMP Shell (CLI)

If you're using the shell, you can add an Extended Document like this:

s-ramp> connect http://localhost:8080/s-ramp-server admin admin
Successfully connected to S-RAMP endpoint: http://localhost:8080/s-ramp-server
s-ramp> upload .project EclipseProjectDocument
Successfully uploaded an artifact.
-- Core S-RAMP Info --
  Type: EclipseProjectDocument
  Model: ext
  UUID: 3eddad25-5cdc-4024-bf7d-ef51714f512d
  Name: .project
  Derived: false
  Created By: admin
  Created On: 2014-09-24T08:11:24.095-04:00
  Modified By: admin
  Modified On: 2014-09-24T08:11:24.095-04:00
-- Document Info --
  Content Type: application/xml
  Content Size: 564
s-ramp> query "/s-ramp/ext/"
Querying the S-RAMP repository:
        /s-ramp/ext
Atom Feed (1 entries)
  Idx                    Type Name
  ---                    ---- ----
    1  EclipseProjectDocument .project

Alternatively you can add an Extended Artifact like this:

s-ramp> help s-ramp:create
USAGE
s-ramp:create type name [description]

The "create" command creates a new artifact in the
S-RAMP repository, without file content.  The artifact
type and name must be provided, along with an optional
description.

Example usages:
>  s-ramp:create HumanActor bwayne "An actor who also wears a stylish belt."
>  s-ramp:create MyCustomArtifactType "Name of my artifact"
s-ramp> s-ramp:create FooArtifact "My Foo Artifact"
Successfully created the artifact: My Foo Artifact
s-ramp> query "/s-ramp/ext/"
Querying the S-RAMP repository:
        /s-ramp/ext
Atom Feed (2 entries)
  Idx                    Type Name
  ---                    ---- ----
    1  EclipseProjectDocument .project
    2             FooArtifact My Foo Artifact
s-ramp> getMetaData
feed:  uuid:
s-ramp> getMetaData feed:2
Meta Data for: 63be87ec-fbfc-4f45-9d1d-cf267f712074
--------------
-- Core S-RAMP Info --
  Type: FooArtifact
  Model: ext
  UUID: 63be87ec-fbfc-4f45-9d1d-cf267f712074
  Name: My Foo Artifact
  Derived: false
  Created By: admin
  Created On: 2014-09-24T08:12:32.869-04:00
  Modified By: admin
  Modified On: 2014-09-24T08:12:32.869-04:00

User Interface

If you're using the UI, you can only add an Extended Document by uploading a file and giving it your custom artifact type.  This can be done from the Import Artifact(s) dialog.  This dialog allows you to specify the file to upload as well as the artifact type.

From Java (using our S-RAMP Atom Client library)

If you are integrating with the S-RAMP repository from Java, you're probably using the S-RAMP atom client provided by our project (the s-ramp-client library).  If so, you can create an Extended Artifact very easily like this:

        SrampAtomApiClient client = new SrampAtomApiClient("http://localhost:8080/s-ramp-server", "admin", "admin", true);
        ExtendedArtifactType artifact = (ExtendedArtifactType) ArtifactType.ExtendedArtifactType("ArtifactFromClient", false).newArtifactInstance();
        artifact.setName("My Artifact (from client)");
        client.createArtifact(artifact);

If you are trying to add an Extended Document, you can do this:

        SrampAtomApiClient client = new SrampAtomApiClient("http://localhost:8080/s-ramp-server", "admin", "admin", true);
        ExtendedDocument artifact = (ExtendedDocument) ArtifactType.ExtendedDocument("FooDocument").newArtifactInstance();
        artifact.setName("My Foo Document (from client)");
        InputStream content = new FileInputStream("/path/to/file.content");
        try {
            client.uploadArtifact(artifact, content);
        } finally {
            content.close();
        }

Unfortunately there is no central server-side file extension registry that maps file extensions to S-RAMP artifact types.  The reason is that the S-RAMP repository's API (defined by the S-RAMP specification) requires that artifacts be added to an Atom collection that includes the artifact model and type.  So for example artifacts must be POST'd to a URL like the following:

http://localhost:8080/s-ramp-server/s-ramp/wsdl/WsdlDocument

There is no generic collection that provides a way to upload a file and have the server detect its type.

We do use the file extension when a file is uploaded, but only to determine the mime-type of the content, not to determine the artifact type.