SOURCE FROM WHERE I REFFERED:

https://salesforce.stackexchange.com/questions/218915/upload-file-and-save-it-as-attachment-using-lightning-component

In My Apex Class M getting ERROR  Here:

 efa.setBody(EncodingUtil.base64Decode(a.ContentDocument.LatestPublishedVersion.VersionData));

Error is:
Method does not exist or incorrect signature: void base64Decode(Blob) from the type System.EncodingUtil

BELOW IS MY APEX CODE(JUST PASTE IN YOUR ORG and Try)

public with sharing class SendEmailController {
    @AuraEnabled
    public static void sendEmailAction(String eTo, String eCc, String eBcc, String eSubject, String eBody, String eFrom, List<String> attach){
        List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
        
        System.debug('attach--'+attach);
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        
        mail.setToAddresses(new List<String>{eTo});
        
        if(eCc != '' && eCc != null)
            mail.setCcAddresses(new List<String>{eCc});
        
        if(eBcc != '' && eBcc != null)
            mail.setBccAddresses(new List<String>{eBcc});
        
        mail.setSubject(eSubject);
        if(eFrom != null)
            mail.setInReplyTo(eFrom);
        
        List<Messaging.Emailfileattachment> fileAttachments = new List<Messaging.Emailfileattachment>();
        for (ContentDocumentLink a : [SELECT id,ContentDocument.Title,ContentDocument.FileType,ContentDocument.FileExtension,
                                      ContentDocument.LatestPublishedVersionId,ContentDocument.LatestPublishedVersion.VersionData  
                                      FROM ContentDocumentLink WHERE LinkedEntityID =:attach])
        {
            Messaging.Emailfileattachment efa = new Messaging.Emailfileattachment();
            
           
                       
            //a.ContentDocument.LatestPublishedVersion.VersionData is a Base64 String. Converting it into Blob
              
            
            //BELOW LINE SHOWING THE ERROR  
           efa.setBody(EncodingUtil.base64Decode(a.ContentDocument.LatestPublishedVersion.VersionData));
         
            /*ERROR====>
          Method does not exist or incorrect signature: void base64Decode(Blob) from the type System.EncodingUtil
          */      
            
            fileAttachments.add(efa);
        }
        mail.setEntityAttachments(attach);
        mail.setHtmlBody(eBody);
        mails.add(mail);
        System.debug('Email-->>'+mails);
        /*Messaging.SendEmailResult[] results = Messaging.sendEmail(mails);

if (results[0].success)
System.debug('The email was sent successfully.');
else
System.debug('The email failed to send: ' + results[0].errors[0].message);*/
    }
    
    @AuraEnabled
    public static List<OrgWideEmailAddress> getOrgWideEmailAddress(){
        return [SELECT Id, Address, DisplayName from OrgWideEmailAddress];
    }
}


LIGHTNING COMPONENT:SendEmailController

SendEmailController.cmp

<aura:component implements="flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId" access="global" controller="SendEmailController">
<aura:attribute name="to" type="String"/>
<aura:attribute name="cc" type="String"/>
<aura:attribute name="bcc" type="String"/>
<aura:attribute name="subject" type="String"/>
<aura:attribute name="body" type="String"/>
<aura:attribute name="from" type="OrgWideEmailAddress[]" />
<aura:attribute name="files" type="List"/>
<aura:handler name="init" action="{!c.doInit}" value="{!this}"/>

<div class="slds-container--medium">
    <div class="slds-form">
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <lightning:select name="select" label="From:" aura:id="fromId">
                    <option value="">--None--</option>
                    <aura:iteration items="{!v.from}" var="frmAdd">
                        <option text="{!frmAdd.DisplayName}" value="{!frmAdd.Address}"></option>
                    </aura:iteration>
                </lightning:select>
            </div>
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <lightning:input type="email" aura:id="isValid" label="To:" value="{!v.to}" name="toAddress" required="true" messageWhenRangeUnderflow="Please provide valid email"/>
            </div>
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <lightning:input type="email" aura:id="isValid" label="CC:" value="{!v.cc}" name="cc" messageWhenRangeUnderflow="Please provide valid email"/>
            </div>
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <lightning:input type="email" aura:id="isValid" label="BCC:" value="{!v.bcc}" name="bcc" messageWhenRangeUnderflow="Please provide valid email"/>
            </div>  
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <lightning:input type="text" label="Subject:" name="subject" value="{!v.subject}"/>
            </div>
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">
                <label class="slds-form-element__label" >Body:</label>
                <lightning:inputRichText placeholder="Type email body" value="{!v.body}"/>
            </div>
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">    
                 <lightning:fileUpload label="Upload File" multiple="true" accept=".pdf, .png, .txt" recordId="{!v.recordId}" aura:id="multifileUpload" onuploadfinished="{!c.uploadFile}" />
            </div>
        </div>
        <div class="slds-form-element">
            <div class="slds-form-element__control">    
                <lightning:button class="slds-button slds-button--brand" onclick="{!c.sendEmail}">Send</lightning:button>
            </div>
        </div>
    </div>
</div>
</aura:component>


CONTROLLER:



({

    doInit: function(component, event, helper) {
        var action = component.get("c.getOrgWideEmailAddress");
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                component.set("v.from", response.getReturnValue());
            }
            else {
                console.log("Failed with state: " + state);
            }
        });
        $A.enqueueAction(action);
    },
    sendEmail: function(component, event, helper) {
        var to = component.get("v.to");
        var cc = component.get("v.cc");
        var bcc = component.get("v.bcc");
        var subject = component.get("v.subject");
        var body = component.get("v.body");
        var from = component.find("fromId").get("v.value");
        //console.log('files before send--'+JSON.stringify(component.get("v.files")));
        var attachments = component.get("v.files");
        //console.log('Added attachments--'+attachments.length);
        var validEmail = component.find('isValid').reduce(function (validSoFar, inputCmp) {
                inputCmp.showHelpMessageIfInvalid();
            return validSoFar && inputCmp.get('v.validity').valid;
        }, true);
        if(validEmail){
            console.log('attachments---'+JSON.stringify(attachments));
            helper.sendEmailHelper(component, to, cc, bcc, subject, body, from, attachments);
        }
    },
    uploadFile: function(component, event, helper){
        var uploadedFiles = component.get("v.files");
        var singleFile = event.getParam("files");
        //console.log('document Id2-->'+singleFile.get('documentId'));
        for(var i = 0; i < singleFile.length; i++){
            console.log('elements--'+JSON.stringify(singleFile[i].documentId));
            uploadedFiles.push(JSON.stringify(singleFile[i].documentId));
        }
        component.set("v.files", uploadedFiles);
    }
})

HELPER

({
    sendEmailHelper : function(component, getTo, getCc, getBcc, getSubject, getBody, getFrom,getAttachments) {
        var action = component.get("c.sendEmailAction");
        console.log('getAttachments in helper--'+JSON.stringify(getAttachments));
        action.setParams({
            'eTo' : getTo,
            'eCc' : getCc,
            'eBcc': getBcc,
            'eSubject' : getSubject,
            'eBody' : getBody,
            'eFrom' : getFrom,
            'attach' : getAttachments
        });
        
        action.setCallback(this, function(response) {
            var state = response.getState();
            if(state == "SUCCESS")
                alert('Email Send successfully..!!');
            else
                alert('Failed to send Email..!!');
        });
        
        $A.enqueueAction(action);
    }
})