Code for that is

<div class="slds-form-element">
    <div aura:id = "uploadBtn" class="slds-form-element__control">
        <div class="slds-file-selector slds-file-selector_files">
            <div class="slds-file-selector__dropzone">
                <input type="file" aura:id="fileId" class="slds-file-selector__input slds-assistive-text" id="file-upload-input-01" aria-describedby="file-selector-id" multiple=""/>
                <label class="slds-file-selector__body" for="file-upload-input-01">
                    <span class="slds-file-selector__button slds-button slds-button_neutral">
                        <lightning:icon class="uploadIcon" iconName="utility:upload" size="xx-small" alternativeText="Icon"/>Attach File&nbsp;&nbsp;&nbsp;&nbsp;</span>
                </label>
            </div>
        </div>
    </div>
    <div aura:id = "fileNameDiv" class = "slds-form-element__control slds-hide">
        <ui:outputText aura:id="fileName"/>
    </div>
    <div lass="slds-form-element__control">
        <lightning:button aura:id="attachBtn" label="Upload Files" iconName="utility:attach" iconPosition="left" onclick="{!c.save}" />
    </div>
</div>

Code for that is

1.Component

<div class="slds-form-element">
    <div aura:id = "uploadBtn" class="slds-form-element__control">
        <div class="slds-file-selector slds-file-selector_files">
            <div class="slds-file-selector__dropzone">
                <input type="file" aura:id="fileId" class="slds-file-selector__input slds-assistive-text" id="file-upload-input-01" aria-describedby="file-selector-id" multiple=""/>
                <label class="slds-file-selector__body" for="file-upload-input-01">
                    <span class="slds-file-selector__button slds-button slds-button_neutral">
                        <lightning:icon class="uploadIcon" iconName="utility:upload" size="xx-small" alternativeText="Icon"/>Attach File&nbsp;&nbsp;&nbsp;&nbsp;</span>
                </label>
            </div>
        </div>
    </div>
    <div aura:id = "fileNameDiv" class = "slds-form-element__control slds-hide">
        <ui:outputText aura:id="fileName"/>
    </div>
    <div lass="slds-form-element__control">
        <lightning:button aura:id="attachBtn" label="Upload Files" iconName="utility:attach" iconPosition="left" onclick="{!c.save}" />
    </div>
</div>

2.Controller

save : function(component, event, helper) {
        helper.save(component,event);
        //console.log(component.get("v.parentId"));
    },

3. Helper
   

   save : function(component,event) { var self = this; var saveElement = event.getSource(); var index = self.getClickedIndex(component,saveElement); console.log('index is'+index); if(index!=-1){ var outlets = component.get("v.outlets"); var parentId = outlets[index].Id; console.log(parentId); //making index as 0 var attachIndex= 0; var fileInput = component.find("fileId")[attachIndex].getElement();

        var file = fileInput.files[attachIndex];
        console.log('File is '+file);
   
        if(file != undefined){
            if (file.size > this.MAX_FILE_SIZE) {
                alert('File size cannot exceed ' + this.MAX_FILE_SIZE + ' bytes.\n' +
                      'Selected file size: ' + file.size);
                return;
            }
   
            var hidebtn = component.find("uploadBtn")[index];
            $A.util.toggleClass(hidebtn, "slds-hide");
            var showFileName = component.find("fileNameDiv")[index];
            var fileNameText = component.find("fileName")[index].set("v.value",file.name);
            $A.util.toggleClass(showFileName, "slds-hide");
   
            var fr = new FileReader();
            var self = this;
   
            //console.log(file.size);
   
            fr.onload = $A.getCallback(function() {
                var fileContents = fr.result;
                var base64Mark = 'base64,';
                var dataStart = fileContents.indexOf(base64Mark) + base64Mark.length;
   
                fileContents = fileContents.substring(dataStart);
   
                self.upload(component, file, fileContents, parentId);
            });
   
            fr.readAsDataURL(file);
        } else {
            alert('File not uploaded');
        }
    }
   

   },

   upload: function(component, file, fileContents, parentId) { var fromPos = 0; var toPos = Math.min(fileContents.length, fromPos + this.CHUNK_SIZE);

    // start with the initial chunk
    this.uploadChunk(component, file, fileContents, fromPos, toPos, '', parentId);   
   

   },

   uploadChunk : function(component, file, fileContents, fromPos, toPos, attachId, parentRecordId) { var action = component.get("c.saveTheChunk"); var chunk = fileContents.substring(fromPos, toPos);

    action.setParams({
        parentId: parentRecordId,
        fileName: file.name,
        base64Data: encodeURIComponent(chunk), 
        contentType: file.type,
        fileId: attachId
    });
   
    var self = this;
    action.setCallback(this, function(a) {
        attachId = a.getReturnValue();
   
        fromPos = toPos;
        toPos = Math.min(fileContents.length, fromPos + self.CHUNK_SIZE);    
        if (fromPos < toPos) {
            self.uploadChunk(component, file, fileContents, fromPos, toPos, attachId);  
        }
    });
    $A.enqueueAction(action); 
   

   },

   //This function is used to get the index of the clicked button for saving the attachment getClickedIndex: function(component, btnElement) { var noOfOutlets = component.get("v.outlets").length;

    for(var i=0; i<noOfOutlets; i++){
        if(component.find("attachBtn")[i]==btnElement) return i;
    }
    return -1;
   

   },