Only set the header if the node exists and in case the request is a GET

apps/dav/lib/server.php

@@ -110,10 +110,15 @@ class Server {
 
 		// Serve all files with an Content-Disposition of type "attachment"
 		$this->server->on('beforeMethod', function (RequestInterface $requestInterface, ResponseInterface $responseInterface) {
-				$node = $this->server->tree->getNodeForPath($requestInterface->getPath());
-				if (($node instanceof IFile)) {
-					$responseInterface->addHeader('Content-Disposition', 'attachment');
+			if ($requestInterface->getMethod() === 'GET') {
+				$path = $requestInterface->getPath();
+				if ($this->server->tree->nodeExists($path)) {
+					$node = $this->server->tree->getNodeForPath($path);
+					if (($node instanceof IFile)) {
+						$responseInterface->addHeader('Content-Disposition', 'attachment');
+					}
 				}
+			}
 		});
 
 		// wait with registering these until auth is handled and the filesystem is setup