When this field is there, the webscript does not get called. Due to this the created by and created date property is unset. When this field is removed from share-config definition, the webscript gets executed. But we want this field definition.
At which time your rule will be triggerred(create ,update etc)?Share-config contains the ui configuration of the forms.So there should not be any impact of it on rule.
One more thing, You have defined hidden.ftl for cm:content field.It does not make sense because, even if you remove this configuration, form will look as it is.It will look save if you include field with hidden.ftl as well.
You can set the mimetype inside the rule.Mimetype is getting decided on the value of cm:content.as hidden.ftl will contain the "" string value it is setting as text/plain.Set the mimtype inside the rule.