[Zope] Confused about permissions

[Brenda Bell]

I have a simple use case for Zope where a group of users need to be able 
to manage content stored in external files.  I installed ExternalFile 
and it seems to be working as expected for the admin user.

I've created a folder where all ExternalFile objects reside.  I've 
defined a local role "Editor" for that folder, added a user and assigned 
that user to the Editor role.

What I can't seem to do is get the permissions right such that a user in 
the Editor role can add new External Files.

I've checked every box on the Security tab for the Editor role.  Users 
in the Editor role can view and modify existing ExternalFile's but still 
cannot add new ones.

I thought maybe the problem had to do with permissions on the 
ExternalFile product itself, but when reading the description on the 
Define Permissions tab I couldn't really figure out what it was trying 
to say:

"The first column below lists the permissions for this object. The 
second specifies the permissions that should have this permission in 
this product or ZClass. For ZClass methods, only permissions that are 
defined for the ZClass are permitted.
In general, any permissions that include operations that change (mutate) 
an object should be disabled."

What exactly does this mean?

More to the point:  What permissions do I need to assign to my Editor 
role so users can add External Files?