Keep the old behaviour for unkown content types

For unknown content-types use the same logic as for
application/vnd.ms-azure-apim.xsd+xml and others. In case new content types will
appear, there is some chance, that this will be desired behaviour.

If future prove us wrong, then it will be easy to identify place requiring
change due to WARN logging.

azurerm/internal/services/apimanagement/api_management_api_schema_resource.go

@@ -178,7 +178,8 @@ func resourceApiManagementApiSchemaRead(d *pluginsdk.ResourceData, meta interfac
 			case "application/vnd.ms-azure-apim.xsd+xml", "application/vnd.ms-azure-apim.wadl.grammars+xml":
 				d.Set("value", documentProperties.Value)
 			default:
-				return fmt.Errorf("[FATAL] Unknown content type %q for schema %q (API Management Service %q / API %q / Resource Group %q)", *properties.ContentType, schemaID, serviceName, apiName, resourceGroup)
+				log.Printf("[WARN] Unknown content type %q for schema %q (API Management Service %q / API %q / Resource Group %q)", *properties.ContentType, schemaID, serviceName, apiName, resourceGroup)
+				d.Set("value", documentProperties.Value)
 			}
 		}
 	}