Don't allow users to populate file custom fields

JonGold requested to merge JonGold/action-provider:file-custom-fields into master

When creating a contact/activity/etc., attempting to map an input to a file custom field results in an error. We can work around this with actions like "Activity: Upload a file to a custom field" - but we shouldn't allow users to map files to custom fields at all.

This patch hides file custom fields when mapping inputs in actions like "Create or Update an Individual".

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