The Apex class to send out the email with the attachments

DownloadAttachments.cls

global class DownloadAttachments {
 /**
  * Send out the startup object attachments to the specified email and save it as the activity
  * @param {String} startupId The Id of the target startup object, which files we should attach to the email
  * @param {String} emailAddress The email address of the recipient
  */
  global static void download(String startupId, String emailAddress) {
    Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage();
    // Set the recipient address
    message.setToAddresses(new String[] { emailAddress });
    // Attach files to the message
    List<Startup__c> startups = [SELECT Id, (SELECT Id FROM Attachments) FROM Startup__c WHERE Id = :startupId];
    if (!startups.isEmpty()) {
      Set<Id> attachmentIds = (new Map<Id, SObject>(startups[0])).keySet();
      List<Attachment> files = [SELECT Name, Body, ContentType FROM Attachment WHERE Id IN :attachmentIds];
    }
    List<Messaging.EmailFileAttachment> attachments = new List<Messaging.EmailFileAttachment>();
    for (Attachment file: files) {
      Messaging.EmailFileAttachment efa = new Messaging.EmailFileAttachment();
      efa.setFileName(file.Name);
      efa.setBody(file.Body);
      efa.setContentType(file.ContentType);
      attachments.add(efa);
    }
    message.setFileAttachments(attachments);
    // Set the message template
    List<EmailTemplate> templates = [SELECT Id, Subject, HtmlValue FROM EmailTemplate WHERE DeveloperName = 'InvestorMaterial'];
    if (!templates.isEmpty()) {
      message.setTemplateId(template[0].Id);
      message.setSubject(template[0].Subject);
      message.setHtmlBody(template[0].HtmlValue);
    }
    // Set the message sender address
    List<OrgWideEmailAddress> addresses = [SELECT Id FROM OrgWideEmailAddress WHERE Address = '[email protected]'];
    if (!addresses.isEmpty()) {
      message.setOrgWideEmailAddressId(addresses[0].Id);
    }
    // Save the message as activity
    List<Lead> leads = [SELECT Id FROM Lead WHERE Email = :emailAddress];
    if (!leads.isEmpty()) {
      message. setTargetObjectId(leads[0].Id);
      message.setSaveAsActivity(true);
    }
    // Send the message
    try {
      Messaging.sendEmail(new Messaging.SingleEmailMessage[] { message });
    } catch (Exception e) {
      throw e;
    }
  }
}