HPGL export works now.

app.py

@@ -243,8 +243,11 @@ def hpgl (id):
     svg = ascii2svg(ascii_input, params['weight'])
 
     # store as a temporary file
-    (svg_file, svg_path) = tempfile.mkstemp()
+    (svg_file, svg_path) = tempfile.mkstemp('.svg')
-    svg_file.write(svg)
+
+
+    with open(svg_file, 'w') as svg_handle:
+        svg_handle.write(svg)
 
     # transform to hpgl
     hpgl = svgToHPGL(svg_path)