fix broken language mode after saving untitled (fix #86971)

src/vs/workbench/services/textfile/browser/textFileService.ts

@@ -433,7 +433,12 @@ export abstract class AbstractTextFileService extends Disposable implements ITex
 				modelToRestoreResource = joinPath(target, sourceModelResource.path.substr(source.path.length + 1));
 			}
 
-			const modelToRestore: ModelToRestore = { resource: modelToRestoreResource, encoding: sourceModel.getEncoding(), mode: sourceModel.textEditorModel?.getModeId() };
+			let mode: string | undefined = sourceModel.textEditorModel?.getModeId();
+			if (mode === PLAINTEXT_MODE_ID) {
+				mode = undefined; // never enforce plain text mode when moving as it is unspecific
+			}
+
+			const modelToRestore: ModelToRestore = { resource: modelToRestoreResource, encoding: sourceModel.getEncoding(), mode };
 			if (sourceModel.isDirty()) {
 				modelToRestore.snapshot = sourceModel.createSnapshot();
 			}
@@ -766,7 +771,12 @@ export abstract class AbstractTextFileService extends Disposable implements ITex
 				await this.create(target, '');
 			}
 
-			targetModel = await this.models.loadOrCreate(target, { encoding: sourceModel.getEncoding(), mode: sourceModel.textEditorModel?.getModeId() });
+			let mode: string | undefined = sourceModel.textEditorModel?.getModeId();
+			if (mode === PLAINTEXT_MODE_ID) {
+				mode = undefined; // never enforce plain text mode when moving as it is unspecific
+			}
+
+			targetModel = await this.models.loadOrCreate(target, { encoding: sourceModel.getEncoding(), mode });
 		}
 
 		try {