fix(actions): drop untrusted Content-Encoding from v1 artifact uploads (#39855)

Legacy (v1 to v3) artifact uploads stored the request's
`Content-Encoding` header in the column that v4 artifacts read as their
`Content-Type`.

- Store only the `gzip` marker on legacy uploads, the only encoding
their clients send
- Pick the PDF and audio/video serve headers only for content types that
`mime.ParseMediaType` accepts

---------

Co-authored-by: silverwind <me@silverwind.io>

tests/integration/api_actions_artifact_test.go

@@ -210,9 +210,11 @@ func TestActionsArtifactUploadSingleFile(t *testing.T) {
 	req = NewRequestWithBody(t, "PUT", url, strings.NewReader(body)).
 		AddTokenAuth("8061e833a55f6fc0157c98b883e91fcfeeb1a71a").
 		SetHeader("Content-Range", "bytes 0-1023/1024").
+		SetHeader("Content-Encoding", "video/mp4,text/html").
 		SetHeader("x-tfs-filelength", "1024").
 		SetHeader("x-actions-results-md5", "XVlf820rMInUi64wmMi6EA==") // base64(md5(body))
 	MakeRequest(t, req, http.StatusOK)
+	assert.Empty(t, unittest.AssertExistsAndLoadBean(t, &actions_model.ActionArtifact{RunID: 791, ArtifactName: "artifact", ArtifactPath: "abc-2.txt"}).ContentEncodingOrType)
 
 	t.Logf("Create artifact confirm")