[SalesForce] Display an attachment in a Lightning component

You can certainly create a sobject instance in client and pass as it as array from the component and retrieve it as List<sObject> in the controller.

If you want to pass a specific sObject type you, have to set the sobjectType.

For eg: sobjectType to Account

Here an simple example to do it.

TestApp:

<aura:application controller="AccountController" access="public">
    <aura:attribute name="accounts" type="List" access="private"/>
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>

    <aura:iteration items="{!v.accounts}" var="acc">
        <ui:inputText value="{!acc.Name}"/><br/>
    </aura:iteration>

    <ui:button press="{!c.saveAcc}" label="Save"/>
</aura:application>

TestAppController.js

({
    doInit : function(cmp, event, helper) {
        var acc = [];

        for(var i = 0;i < 5;i++){
            //set sobjectType to Account, if account is passed
            acc.push({'sobjectType':'Account','Name':'Test'+i});
        }

        cmp.set("v.accounts",acc);

    },
    saveAcc : function(cmp, event, helper) {

        var action = cmp.get("c.insertAccounts");
        action.setParams({
            "acc":cmp.get("v.accounts")
        });

        actions.setCallback(this,function(resp){

            var state = resp.getState();

            if(state === 'SUCCESS'){
                console.log(resp.getReturnValue());
            }
            else if(state === 'ERROR'){
                var errors = resp.getError();
                for(var i = 0 ;i < errors.length;i++){
                    console.log(errors[i].message);
                }
            }

        });

        $A.enqueueAction(action);

    }
})

Apex Controller:

public with sharing class AccountController {

    //if sobject is specific for eg:Account,then you use List<Account> acc
    @AuraEnabled
    public static String insertAccounts(List<sObject> acc){
        try{
            insert acc;
            System.debug('acc'+acc);
            return 'success';
        }
        catch(DMLException ex){
           AuraHandledException e = new AuraHandledException(ex.getMessage());
           throw e;
        }
        return '';        
    }
}

1. On your apex server side code you need to select also the attachment object -> all the attachments of the selected objects ids.
2. Create a wrapper apex class that will represent 1 record with a list of attachments and prepare your data so each record has a list of attachments. Then send a list of this wrapper class back to the client (you can do this also on the client side javascript and not from the server).

3. on your client side markup you should define an attribute from the wrapper class, and have an aura:iteration for each record, and inside another aura:iteration for the attachments in each record:

   <aura:attribute type="ControllerClass.wrapperClass[]" name="records" />
   

4. Displaying the downloadable links - you can do in many ways - for example, I display download links like that (where att is the iteration variable):

   <div class="slds-media__body">
       <h3 class="slds-truncate" title="File Name">
           <a href="{!'/servlet/servlet.FileDownload?file='+att.Id}">{!att.Name}</a>
       </h3>
       <div class="slds-tile__detail slds-text-body--small">
           <ul class="slds-list--horizontal slds-has-dividers--right">
               <li class="slds-item">
                   <ui:outputDate format="dd/MM/yyyy HH:mm" value="{!att.LastModifiedDate}"/>
               </li>
               <li class="slds-item">
                   KB&nbsp;<ui:outputNumber format="#,###,###.#" value="{!(att.BodyLength) / 1000}" />
               </li>
           </ul>
       </div>
   </div>