Custom File Upload Using LWC

You will learn the following things

How to fetch the file details
How to fetch the record id
How to avoid specific file type
How to send a data stream to Apex
How apex catch the file from lwc and store it.
How to handle success message using toast

Video Tutorial

Steps and code

Create Lwc component fileUploaderCompLwc and add the following code to the respective files.

fileUploaderCompLwc.html

<template>
    <lightning-card title="File Upload Demo LWC" icon-name="custom:custom14">
        <div class="slds-m-around_medium">
            <lightning-input type="file" 
            accept=".xlsx, .xls, .csv, .png, .doc, .docx, .pdf"
            label="Attachment" onchange={openfileUpload}></lightning-input>
        </div>
        <template if:true={fileData}>
            <p>{fileData.filename}</p>
        </template>
        <lightning-button variant="brand" label="submit" title="Submit" onclick={handleClick} class="slds-m-left_x-small"></lightning-button>
    </lightning-card>
</template>

In the fileUploaderCompLwc.html we are creating the card component and within the card creating two elements, lightning-input and lightning-button
We are using lightning-input of type file which will allow us to choose a file from the system.
In the input field, we are using the file format that will be only visible to the user while uploading the file
Once the File upload, we call the onchange event that will call openfileupload handler.
Once the file gets uploaded to javascript will extract the filename and show on the HTML
We have lightning-button of type submit onclick of which we are calling the apex method.

fileUploaderCompLwc.js

import { LightningElement, api } from 'lwc';
import { ShowToastEvent } from 'lightning/platformShowToastEvent';
import uploadFile from '@salesforce/apex/FileUploaderClass.uploadFile'
export default class FileUploaderCompLwc extends LightningElement {
    @api recordId;
    fileData
    openfileUpload(event) {
        const file = event.target.files[0]
        var reader = new FileReader()
        reader.onload = () => {
            var base64 = reader.result.split(',')[1]
            this.fileData = {
                'filename': file.name,
                'base64': base64,
                'recordId': this.recordId
            }
            console.log(this.fileData)
        }
        reader.readAsDataURL(file)
    }
    
    handleClick(){
        const {base64, filename, recordId} = this.fileData
        uploadFile({ base64, filename, recordId }).then(result=>{
            this.fileData = null
            let title = `${filename} uploaded successfully!!`
            this.toast(title)
        })
    }

    toast(title){
        const toastEvent = new ShowToastEvent({
            title, 
            variant:"success"
        })
        this.dispatchEvent(toastEvent)
    }
}

In fileUploaderCompLwc.js, we are fetching the record id using @api, and this id is required to send the file to the apex.
As soon as the file upload onchange, we call openfileUpload handler that receives an event from which we extract the details of the file like name, size, etc.
Inside openfileUpload method, we use browser FileReader API to generate the data stream.
On click of the submit button, handleClick method gets called, and it calls the apex method uploadFile.
Once the file upload is completed, it will trigger the toast method that shows the screen's success message.

fileUploaderCompLwc.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>48.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__RecordPage</target>
    </targets>
</LightningComponentBundle>

Create an Apex class FileUploaderClass.cls and add the following code to the file.

FileUploaderClass.cls

public with sharing class FileUploaderClass {
    /*
   * @method uploadFile() 
   * @desc Creates a content version from a given file's base64 and name
   * 
   * @param {String} base64 - base64 string that represents the file
   * @param {String} filename - full file name with extension, i.e. 'products.csv'
   * @param {String} recordId - Id of the record you want to attach this file to
   * 
   * @return {ContentVersion} - returns the created ContentDocumentLink Id if the
   *   upload was successful, otherwise returns null
   */
   @AuraEnabled
  public static String uploadFile(String base64, String filename, String recordId) {
        ContentVersion cv = createContentVersion(base64, filename);
        ContentDocumentLink cdl = createContentLink(cv.Id, recordId);
        if (cv == null || cdl == null) { return null; }
        return cdl.Id;
  }
  /*
   * @method createContentVersion() [private]
   * @desc Creates a content version from a given file's base64 and name
   * 
   * @param {String} base64 - base64 string that represents the file
   * @param {String} filename - full file name with extension, i.e. 'products.csv'
   * 
   * @return {ContentVersion} - returns the newly created ContentVersion, or null
   *   if there was an error inserting the record
   */
  private static ContentVersion createContentVersion(String base64, String filename) {
    ContentVersion cv = new ContentVersion();
    cv.VersionData = EncodingUtil.base64Decode(base64);
    cv.Title = filename;
    cv.PathOnClient = filename;
    try {
      insert cv;
      return cv;
    } catch(DMLException e) {
      System.debug(e);
      return null;
    }
  }

   /*
   * @method createContentLink() [private]
   * @desc Creates a content link for a given ContentVersion and record
   * 
   * @param {String} contentVersionId - Id of the ContentVersion of the file
   * @param {String} recordId - Id of the record you want to attach this file to
   * 
   * @return {ContentDocumentLink} - returns the newly created ContentDocumentLink, 
   *   or null if there was an error inserting the record
   */
  private static ContentDocumentLink createContentLink(String contentVersionId, String recordId) {
              if (contentVersionId == null || recordId == null) { return null; }
    ContentDocumentLink cdl = new ContentDocumentLink();
    cdl.ContentDocumentId = [
      SELECT ContentDocumentId 
      FROM ContentVersion 
      WHERE Id =: contentVersionId
    ].ContentDocumentId;
    cdl.LinkedEntityId = recordId;
    // ShareType is either 'V', 'C', or 'I'
    // V = Viewer, C = Collaborator, I = Inferred
    cdl.ShareType = 'V';
    try {
      insert cdl;
      return cdl;
    } catch(DMLException e) {
      System.debug(e);
      return null;
    }
  }
}

Final Output

Now place your component to the record page. You will see the following output

Fig:1 fileUploaderCompLwc component after pacing on the page

Fig:2 Success message after uploading the file successfully