Thanks, Willr. I knew there had to be a good way to debug asynchronous requests...
The issue seems to have to do with the removal of the now deprecated HasManyFileManager. Just to document the issue in case someone else has it, I'll post what I learned below. I think the method in question was added as part of the HasManyFileManager documentation and I forgot to remove it when I upgraded and moved to DataObjectManager.
ERROR [User Error]: Uncaught Exception: Object->__call(): the method 'deleteattachedfiles' does not exist on 'Page'
IN POST /admin/EditForm
Line 576 in /path/to/sapphire/core/Object.php
Source
======
567: case isset($config['function']) :
568: return $config['function']($this, $arguments);
569:
570: default :
571: throw new Exception (
572: "Object->__call(): extra method $method is invalid on $this->class:" . var_export($config,
true)
573: );
574: }
575: } else {
* 576: throw new Exception("Object->__call(): the method '$method' does not exist on '$this->class'");
577: }
578: }
579:
580: //
-----------------------------------------------------------------------------------------------------------------
581:
582: /**
<ul></ul>