Las configuraciones fueron movidas y cambiadas un poco

files/files.py

@@ -6,13 +6,11 @@ from files.auth import admin_required
 
 import random
 
-ALLOWED_EXTENSIONS = set(['png', 'jpg', 'jpeg', 'gif', 'md'])
-
 bp = Blueprint('files', __name__)
 bp.add_url_rule('/uploads/<path:filename>', 'uploaded_file', build_only=True)
 
 def allowed_file(filename):
-    return '.' in filename and filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS
+    return '.' in filename and filename.rsplit('.', 1)[1].lower() in current_app.config['ALLOWED_EXTENSIONS']
 
 @bp.route('/', methods=['GET', 'POST'])             
 def index():