diff --git a/offlineu_core.py b/offlineu_core.py
index 75a95c1..c4c18eb 100644
--- a/offlineu_core.py
+++ b/offlineu_core.py
@@ -725,12 +725,29 @@ def serve_file(filepath):
 
         print(f"Serving file: {full_path}")
 
-        # Determine MIME type
-        mime_type, _ = mimetypes.guess_type(full_path)
-        if mime_type is None:
-            mime_type = 'application/octet-stream'
+        # Determine MIME type. Don't rely solely on mimetypes.guess_type() -
+        # its results can vary by OS/container base image depending on what
+        # system mime databases are present. Force the types that matter for
+        # inline preview (PDF above all) so this can't silently regress.
+        ext = os.path.splitext(full_path)[1].lower()
+        KNOWN_MIME_TYPES = {
+            '.pdf': 'application/pdf',
+            '.html': 'text/html',
+            '.htm': 'text/html',
+            '.txt': 'text/plain',
+            '.md': 'text/plain',
+        }
+        if ext in KNOWN_MIME_TYPES:
+            mime_type = KNOWN_MIME_TYPES[ext]
+        else:
+            mime_type, _ = mimetypes.guess_type(full_path)
+            if mime_type is None:
+                mime_type = 'application/octet-stream'
 
-        return send_file(full_path, mimetype=mime_type)
+        # as_attachment=False (the default) sends Content-Disposition: inline
+        # so the browser renders PDFs/HTML in the iframe instead of prompting
+        # a download. Being explicit here so this can't drift.
+        return send_file(full_path, mimetype=mime_type, as_attachment=False)
     except Exception as e:
         print(f"Error serving file: {str(e)}")
         return f"Error serving file: {str(e)}", 500