Get Custom Object Related Files using Salesforce Apex

GetCustomObjectFiles.cls

public Set<Id> getFiles(Id recordId) {

    List<ContentDocumentLink> files = [SELECT ContentDocumentId  
                                       FROM ContentDocumentLink 
                                       WHERE LinkedEntityId IN ( SELECT Id FROM CUSTOM_OBJECT_NAME__c ) 
                                       AND LinkedEntity.Type='CUSTOM_OBJECT_NAME__c' 
                                       AND LinkedEntityId = :recordId];

    if (files.size() > 0) {
      return (new Map<Id, ContentDocumentLink>(files)).keySet();    
    }

    return new Set<Id>();
}