Check out the example on the link!

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 @AuraEnabled 

    public static EmailMsg getTemplateDetails(string templateId, String whoId, String whatId){

        Messaging.SingleEmailMessage email = Messaging.renderStoredEmailTemplate(templateId, whoId, WhatId,Messaging.AttachmentRetrievalOption.METADATA_ONLY);

        EmailMsg msg = new EmailMsg();

        msg.subject = email.getSubject();

        msg.body = email.getHtmlBody();

​

        List<Messaging.EmailFileAttachment> attachmentList = email.fileAttachments;

​

        List<FileAttachmentWrapper> fawList = new List<FileAttachmentWrapper>();

        for(Messaging.EmailFileAttachment efa : attachmentList){

           FileAttachmentWrapper faw = new FileAttachmentWrapper();

            faw.attachId = efa.id;

            faw.filename = efa.filename;

            faw.isContentDocument=false;

            fawList.add(faw);

        }

        for(ContentDocumentLink cdl : [Select ContentDocument.Id, ContentDocument.title, ContentDocument.fileExtension

                                       from contentdocumentlink

                                       where linkedEntityId=:templateId]){

            FileAttachmentWrapper faw = new FileAttachmentWrapper();

            faw.attachId = cdl.ContentDocument.id;

            faw.isContentDocument = true;                               

            faw.filename = cdl.ContentDocument.title+'.'+cdl.contentdocument.fileextension;

            fawList.add(faw);

        }

        msg.fileattachments = fawList;

        if(String.isblank(msg.body)){

            msg.body = email.getPlainTextBody();

            if(String.isNotBlank(msg.body)){

                msg.body = msg.body.replace('\n', '<br/>');

            }

        }

        return msg;   

    }