Apex File File Upload (Using ContentVersion)

ApexFileUpload.cls

/*
 * @class FileUploaderClass
 * @desc Lets you uplaod a file in Salesforce by giving a base64 string of the
 *   file, a name for the file, and the Id of the record that you want to attach 
 *   the file to. 
 *
 * @example:
 *   FileUploaderClass.uploadFile(myBase64String, 'Invoice.pdf', '906F0000000kG2UIAU')
 */
public 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. 'Invoice.pdf'
   * @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
   */
  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. 'Invoice.pdf'
   * 
   * @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;
    }
  }
  
}