fix: test for page file context in download asset

Author: markdumay

layouts/_partials/assets/download.html

@@ -23,7 +23,7 @@
 {{ if not $error }}
     {{- $download = partial "utilities/GetTargetPath.html" (dict "path" $args.download "page" page) -}}
     {{- if and $download (not (fileExists (path.Join "static" $download))) -}}
-        {{ if page }}
+        {{ if page.File }}
             {{- errorf "Cannot find download file for page '%s': %s" page.File.Path $download -}}
         {{ else }}
             {{- errorf "Cannot find download file: %s" $download -}}