fix(Basic LLM Chain Node): Use correct mimetype for binary data (#12978)

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/ChainLlm.node.ts

@@ -34,6 +34,7 @@ import { getOptionalOutputParsers } from '@utils/output_parsers/N8nOutputParser'
 import { getTemplateNoticeField } from '@utils/sharedFields';
 import { getTracingConfig } from '@utils/tracing';
 
+import { dataUriFromImageData, UnsupportedMimeTypeError } from './utils';
 import {
 	getCustomErrorMessage as getCustomOpenAiErrorMessage,
 	isOpenAiError,
@@ -88,7 +89,9 @@ async function getImageMessage(
 		NodeConnectionType.AiLanguageModel,
 		0,
 	)) as BaseLanguageModel;
-	const dataURI = `data:image/jpeg;base64,${bufferData.toString('base64')}`;
+
+	try {
+		const dataURI = dataUriFromImageData(binaryData, bufferData);
 
 		const directUriModels = [ChatGoogleGenerativeAI, ChatOllama];
 		const imageUrl = directUriModels.some((i) => model instanceof i)
@@ -103,6 +106,11 @@ async function getImageMessage(
 				},
 			],
 		});
+	} catch (error) {
+		if (error instanceof UnsupportedMimeTypeError)
+			throw new NodeOperationError(context.getNode(), error.message);
+		throw error;
+	}
 }
 
 async function getChainPromptTemplate(

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/test/utils.test.ts

@@ -0,0 +1,23 @@
+import { mock } from 'jest-mock-extended';
+import type { IBinaryData } from 'n8n-workflow';
+
+import { dataUriFromImageData, UnsupportedMimeTypeError } from '../utils';
+
+describe('dataUriFromImageData', () => {
+	it('should not throw an error on images', async () => {
+		const mockBuffer = Buffer.from('Test data');
+		const mockBinaryData = mock<IBinaryData>({ mimeType: 'image/jpeg' });
+
+		const dataUri = dataUriFromImageData(mockBinaryData, mockBuffer);
+		expect(dataUri).toBe('data:image/jpeg;base64,VGVzdCBkYXRh');
+	});
+
+	it('should throw an UnsupportetMimeTypeError on non-images', async () => {
+		const mockBuffer = Buffer.from('Test data');
+		const mockBinaryData = mock<IBinaryData>({ mimeType: 'text/plain' });
+
+		expect(() => {
+			dataUriFromImageData(mockBinaryData, mockBuffer);
+		}).toThrow(UnsupportedMimeTypeError);
+	});
+});

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/utils.ts

@@ -0,0 +1,12 @@
+import type { IBinaryData } from 'n8n-workflow';
+import { ApplicationError } from 'n8n-workflow';
+
+export class UnsupportedMimeTypeError extends ApplicationError {}
+
+export function dataUriFromImageData(binaryData: IBinaryData, bufferData: Buffer) {
+	if (!binaryData.mimeType?.startsWith('image/'))
+		throw new UnsupportedMimeTypeError(
+			`${binaryData.mimeType} is not a supported type of binary data. Only images are supported.`,
+		);
+	return `data:${binaryData.mimeType};base64,${bufferData.toString('base64')}`;
+}