If the caller does not specify the fields to include in the request (or explicitly exclude fields), then all the properties of a business object are returned. When you have a business object with large properties like base64Content, you may want to exclude these by default. This is easily accomplished using a Before Access event handler:
if (!query.fields) {
query.fields = { base64Content: false };
}
If the caller does not specify "fields" in the query, the event handler automatically adds the exclusion.