send from directory prefix

postit/generate.py

@@ -1,6 +1,8 @@
 from flask import Blueprint, render_template, send_from_directory
 import json
 from . import dump
+import os
+
 
 bp = Blueprint(
     "generate",
@@ -22,4 +24,6 @@ def postit():
 
 @bp.route("/<slug>/<filename>/")
 def send_static(slug=None, filename=None):
-    return send_from_directory(f"/postit/contents/{slug}", filename)
+    return send_from_directory(
+        f'{os.environ.get("URL_PREFIX")}/postit/contents/{slug}', filename
+    )