diff --git a/internal/oci/oci.go b/internal/oci/oci.go
index e27302e64f4d..4bb9a5d7c1e1 100644
--- a/internal/oci/oci.go
+++ b/internal/oci/oci.go
@@ -158,7 +158,7 @@ func findArtifactDescriptor(all []ocispec.Descriptor, opts downloadOptions) (oci
 		}
 	}
 	if opts.artifactName == "" {
-		return ocispec.Descriptor{}, fmt.Errorf("no artifacts found")
+		return ocispec.Descriptor{}, fmt.Errorf("no artifact descriptors found")
 	}
 	return ocispec.Descriptor{}, fmt.Errorf("artifact %q not found", opts.artifactName)
 }
diff --git a/internal/oci/oci_test.go b/internal/oci/oci_test.go
index aecd65393ae5..e2fbfeed0bb9 100644
--- a/internal/oci/oci_test.go
+++ b/internal/oci/oci_test.go
@@ -18,6 +18,7 @@ package oci_test
 
 import (
 	"bytes"
+	"cmp"
 	"context"
 	"embed"
 	"encoding/json"
@@ -61,15 +62,16 @@ func parseTestsYAML[T any](t *testing.T) map[string]T {
 
 // testFile represents a single test file inside the testdata directory.
 type testFile struct {
-	Manifest      string            `json:"manifest"`
-	Expected      string            `json:"expected"`
-	Error         string            `json:"error"`
-	Authenticated bool              `json:"authenticated"`
-	AuthUser      string            `json:"authUser"`
-	AuthPass      string            `json:"authPass"`
-	Artifacts     map[string]string `json:"artifacts"`
-	ArtifactName  string            `json:"artifactName"`
-	PlainHTTP     bool              `json:"plainHTTP"`
+	Manifest          string            `json:"manifest"`
+	ManifestMediaType string            `json:"manifestMediaType"`
+	Expected          string            `json:"expected"`
+	Error             string            `json:"error"`
+	Authenticated     bool              `json:"authenticated"`
+	AuthUser          string            `json:"authUser"`
+	AuthPass          string            `json:"authPass"`
+	Artifacts         map[string]string `json:"artifacts"`
+	ArtifactName      string            `json:"artifactName"`
+	PlainHTTP         bool              `json:"plainHTTP"`
 }
 
 func TestDownload(t *testing.T) {
@@ -170,7 +172,7 @@ func startOCIMockServer(t *testing.T, tname string, test testFile) string {
 
 			// serve the manifest.
 			if strings.Contains(r.URL.Path, "/manifests/") {
-				w.Header().Add("Content-Type", "application/vnd.oci.image.manifest.v1+json")
+				w.Header().Add("Content-Type", cmp.Or(test.ManifestMediaType, "application/vnd.oci.image.manifest.v1+json"))
 				_, _ = w.Write([]byte(test.Manifest))
 				return
 			}
diff --git a/internal/oci/testdata/bad-manifest.yaml b/internal/oci/testdata/bad-manifest.yaml
new file mode 100644
index 000000000000..a4d856e1bc08
--- /dev/null
+++ b/internal/oci/testdata/bad-manifest.yaml
@@ -0,0 +1,3 @@
+manifest: I am most definitely not an OCI manifest!
+error: >-
+  failed to fetch successors: invalid character 'I' looking for beginning of value
diff --git a/internal/oci/testdata/no-artifacts.yaml b/internal/oci/testdata/no-artifacts.yaml
index 7f229dab8182..132ad4ef7928 100644
--- a/internal/oci/testdata/no-artifacts.yaml
+++ b/internal/oci/testdata/no-artifacts.yaml
@@ -15,4 +15,5 @@ manifest: |
   }
 artifacts:
   44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a: "{}"
-error: no artifacts found
+error: >-
+  failed to find artifact: no artifact descriptors found
diff --git a/internal/oci/testdata/not-a-manifest.yaml b/internal/oci/testdata/not-a-manifest.yaml
new file mode 100644
index 000000000000..42c46fe43eed
--- /dev/null
+++ b/internal/oci/testdata/not-a-manifest.yaml
@@ -0,0 +1,6 @@
+manifest: I am most definitely not an OCI manifest!
+manifestMediaType: text/plain
+# TODO: This seems like a misleading error message.
+# Maybe add an extra check in the code for "this is not a manifest"?
+error: >-
+  failed to find artifact: no artifact descriptors found