From 86d3eeb51fd78b7e3438468db018c49b744de6a3 Mon Sep 17 00:00:00 2001
From: Martin Porwoll <martin.porwoll@complexcaresolutions.de>
Date: Sun, 1 Mar 2026 10:36:09 +0000
Subject: [PATCH] fix: infer MIME type from file extension for Telegram
 downloads
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Telegram's file API returns application/octet-stream instead of the
actual image type. Added inferMimeType() that detects the real type
from the URL extension (jpg→image/jpeg, png→image/png, etc.) when
the server response Content-Type is generic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 src/utils/download.ts | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/src/utils/download.ts b/src/utils/download.ts
index 24239b0..0ec6351 100644
--- a/src/utils/download.ts
+++ b/src/utils/download.ts
@@ -3,6 +3,29 @@ import { createLogger } from './logger.js';
 const log = createLogger('Download');
 const MAX_FILE_SIZE = 20 * 1024 * 1024; // 20 MB
 
+// Telegram's file API often returns application/octet-stream — infer from extension
+const EXT_MIME_MAP: Record<string, string> = {
+  jpg: 'image/jpeg',
+  jpeg: 'image/jpeg',
+  png: 'image/png',
+  gif: 'image/gif',
+  webp: 'image/webp',
+  avif: 'image/avif',
+  svg: 'image/svg+xml',
+  pdf: 'application/pdf',
+  mp4: 'video/mp4',
+};
+
+function inferMimeType(url: string, headerType: string): string {
+  // If server returned a specific image/video/pdf type, trust it
+  if (headerType && !headerType.startsWith('application/octet-stream') && headerType !== 'text/plain') {
+    return headerType.split(';')[0].trim();
+  }
+  // Infer from URL extension
+  const ext = url.split(/[?#]/)[0].split('.').pop()?.toLowerCase() ?? '';
+  return EXT_MIME_MAP[ext] ?? headerType ?? 'application/octet-stream';
+}
+
 export async function downloadFile(url: string): Promise<{ buffer: Buffer; mimeType: string }> {
   const controller = new AbortController();
   const timeout = setTimeout(() => controller.abort(), 30_000);
@@ -23,8 +46,9 @@ export async function downloadFile(url: string): Promise<{ buffer: Buffer; mimeT
       throw new Error(`Datei zu groß (${(arrayBuffer.byteLength / 1024 / 1024).toFixed(1)} MB). Maximum: 20 MB`);
     }
 
-    const mimeType = response.headers.get('content-type') || 'application/octet-stream';
-    log.debug(`Downloaded ${arrayBuffer.byteLength} bytes, type: ${mimeType}`);
+    const rawType = response.headers.get('content-type') || 'application/octet-stream';
+    const mimeType = inferMimeType(url, rawType);
+    log.info(`Downloaded ${arrayBuffer.byteLength} bytes, header: ${rawType}, resolved: ${mimeType}`);
 
     return { buffer: Buffer.from(arrayBuffer), mimeType };
   } catch (error) {