Is there a submit handler for custom fields? I built a custom field, and now I'm adding an image upload to that custom field.  The type is managed_file.  When the user saves the content, where can I handle the file after submit?
You can use hook_field_update in your module where you define your field..


  Define custom update behavior for this module's field data.
  
  This hook is invoked from field_attach_update() on the module that
  defines a field, during the process of updating an entity object
  (node, taxonomy term, etc.). It is invoked just before the data for
  this field on the particular entity object is updated into field
  storage. Only field modules that are storing or tracking information
  outside the standard field storage mechanism need to implement this
  hook.