diff --git a/src/api_package/global.d.ts b/src/api_package/global.d.ts
index ccd66138f..119493447 100644
--- a/src/api_package/global.d.ts
+++ b/src/api_package/global.d.ts
@@ -12,5 +12,11 @@ declare global {
 		 */
 		// eslint-disable-next-line camelcase
 		_oca_viewer_handlers: Map<string, IHandler>
+
+		OCA?: {
+			Viewer?: {
+				registerHandler?: (handler: IHandler) => void
+			}
+		}
 	}
 }
diff --git a/src/api_package/index.ts b/src/api_package/index.ts
index bdb215809..58c7f9159 100644
--- a/src/api_package/index.ts
+++ b/src/api_package/index.ts
@@ -44,8 +44,9 @@ export interface IHandler {
 
 /**
  * Register a new handler for the viewer.
- * This needs to be called before the viewer is initialized to ensure the handler is available.
- * So this should be called from an initialization script (`OCP\Util::addInitScript`).
+ * Should be called from an initialization script (`OCP\Util::addInitScript`).
+ * If the ViewerService is already initialized the handler is registered
+ * immediately, otherwise it will be picked up on DOMContentLoaded.
  *
  * @param handler - The handler to register
  * @throws Error if the handler is invalid
@@ -60,6 +61,15 @@ export function registerHandler(handler: IHandler): void {
 	}
 
 	window._oca_viewer_handlers.set(handler.id, handler)
+
+	// If the ViewerService is already initialized, register the handler
+	// directly so its mimetypes are available immediately for file actions.
+	// Without this, mimetypes are only bridged on DOMContentLoaded which
+	// can race with the file list rendering — causing downloads instead of
+	// opening files in the viewer.
+	if (window.OCA?.Viewer?.registerHandler) {
+		window.OCA.Viewer.registerHandler(handler)
+	}
 }
 
 /**