specs-go: clarify mediatypes

Due to the conflicting use of the `mediatType` field across
documents, and after discussion on
opencontainers#411,
this changeset removes the use of `mediaType` where it is used to refers
to a document's own type. Leaving only the use of `mediaType` for
descriptors, where it is used to describe the type of a referenced object.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

specs-go/v1/config.go

@@ -78,6 +78,7 @@ type History struct {
 }
 
 // Image is the JSON structure which describes some basic information about the image.
+// This provides the `application/vnd.oci.image.config.v1+json` mediatype when marshalled to JSON.
 type Image struct {
 	// Created defines an ISO-8601 formatted combined date and time at which the image was created.
 	Created string `json:"created,omitempty"`

specs-go/v1/descriptor.go

@@ -15,9 +15,10 @@
 package v1
 
 // Descriptor describes the disposition of targeted content.
+// This structure provides `application/vnd.oci.descriptor.v1+json` mediatype when marshalled to JSON
 type Descriptor struct {
-	// MediaType contains the MIME type of the referenced object.
-	MediaType string `json:"mediaType"`
+	// MediaType is the media type of the object this schema refers to.
+	MediaType string `json:"mediaType,omitempty"`
 
 	// Digest is the digest of the targeted content.
 	Digest string `json:"digest"`

specs-go/v1/manifest.go

@@ -16,7 +16,7 @@ package v1
 
 import "github.com/opencontainers/image-spec/specs-go"
 
-// Manifest defines a schema2 manifest
+// Manifest provides `application/vnd.oci.image.manifest.list.v1+json` mediatype structure when marshalled to JSON.
 type Manifest struct {
 	specs.Versioned
 

specs-go/v1/manifest_list.go

@@ -50,7 +50,8 @@ type ManifestDescriptor struct {
 	Platform Platform `json:"platform"`
 }
 
-// ManifestList  references manifests for various platforms.
+// ManifestList references manifests for various platforms.
+// This structure provides `application/vnd.oci.image.manifest.list.v1+json` mediatype when marshalled to JSON.
 type ManifestList struct {
 	specs.Versioned
 

specs-go/versioned.go

@@ -20,7 +20,4 @@ package specs
 type Versioned struct {
 	// SchemaVersion is the image manifest schema that this image follows
 	SchemaVersion int `json:"schemaVersion"`
-
-	// MediaType is the media type of this schema.
-	MediaType string `json:"mediaType"`
 }