[SalesForce] How to retrieve the file content for a ContentDocument object

Files are stored in the ContentDocument Object .You will have to run a SOQL against contentDocument link first and look for all Content Version DocumentLink with EntityId as the ParentId of the record .

SELECT ContentDocumentId FROM ContentDocumentLink WHERE LinkedEntityId = '[RECORD ID]'

Then SOQL on ContentDocument

Select Id ,Title from ContentDocument Where ID In :[CONTENTDOCUMENTLIST]

Looking at the Data Model of the Content Object as mentioned in this post, it looks like you need to get Id of the ContentVersion which is tied to you the ContentDocument record. Also to display Contents, the relative url path should be in below format :

/sfc/servlet.shepherd/version/download/contentversionid

Sample code

myApp.app:

<aura:application extends="force:slds" controller="TestCtrl">
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" /> 
    <aura:attribute name="contents" type="List" default="[]"/>
    <aura:attribute name="prefixURL" type="String" default="/sfc/servlet.shepherd/version/download/"/>
    <aura:iteration items="{!v.contents}" var="content">
        <img src="{!v.prefixURL + content.Id}" width="100" height="100"/>
    </aura:iteration>
</aura:application>

controller.js

({
    doInit : function(cmp){
       var action = cmp.get("c.getContents");
       action.setCallback(this, function(response) {
           var state = response.getState();
           if(cmp.isValid() && state === 'SUCCESS') {
               cmp.set("v.contents", response.getReturnValue()); 
           }
       });
        $A.enqueueAction(action);
    },

})

Apex Class:

public class TestCtrl {

    @AuraEnabled
    public static List<ContentVersion> getContents() {
        Set<Id> contentIds = new Set<Id>();

        for (ContentDocument cont :[Select Id ,Title from ContentDocument]) {
            contentIds.add(cont.Id);
        }

        return [SELECT Id,Title FROM ContentVersion WHERE ContentDocumentId IN :contentIds];
    }
}