Custom blocks with managed file fields result in Uncaught Ajax Error 500

When trying to place a custom block with a managed file in it’s block configuration form, panels is not able to store the file object.

Tests to reproduce:
1. Enable panelizer, panels, ctools_views
2. Panelize a content type (e.g. “Landing Page”)
3. Create an panelized entity
4. Navigate to your panelized page
5. Click “Place Content”
6. Select your custom block with a managed file.
7. On the block configuration form (within IPE), click browse and select a file (Note: You’ll also notice the “Upload” button is not present).
8. Verify the following ajax error in JS console:

Uncaught AjaxError: 
An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /admin/panels_ipe/variant/panelizer_field/node%3A1162%3Afull/layout/radix_boxton/block_plugins/custom_block/form?element_parents=flipper/front/settings/file_upload&_wrapper_format=drupal_ajax&ajax_form=1
StatusText: Internal Server Error
ResponseText: {"message":"A fatal error occurred: Serialization of \u0027Symfony\\Component\\HttpFoundation\\File\\UploadedFile\u0027 is not allowed"}