public without sharing class FileUploadController

@AuraEnabled
public static Id saveTheFile(Id parentId, String fileName, String base64Data, String contentType) {

    try {

        base64Data = EncodingUtil.urlDecode(base64Data, 'UTF-8');

        ContentVersion contentVersion = new ContentVersion(
            versionData = EncodingUtil.base64Decode(base64Data),
            title = fileName,
            pathOnClient =  '/' + fileName,
            FirstPublishLocationId = parentId);

        insert contentVersion;

        return contentVersion.Id;

    } catch (Exception error) {
        throw new AuraHandledException(error.getMessage());
    }
}

private static void appendToFile(Id fileId, String base64Data) {

    base64Data = EncodingUtil.urlDecode(base64Data, 'UTF-8');

    ContentVersion contentVersion = [
        SELECT Id, versionData
        FROM ContentVersion
        WHERE Id = :fileId
    ];

    String existingBody = EncodingUtil.base64Encode(contentVersion.versionData);

    contentVersion.versionData = EncodingUtil.base64Decode(existingBody + base64Data);

    update contentVersion;
}