How SharePoint Framework Workbench.aspx page getting local files in Office365 SharePoint Online

This below code sample shows how get local files into  SharePoint workbench page in Office 365. The SharePoint Framework (SPFx) is a page and web part model that provides full support for client-side SharePoint development, easy integration with SharePoint data and support for open source tooling. Import this below namespace in your solution .

Since you are running Gulp on Node and have to load "unsafe script" in your browser you are accepting the script source as your local machine.

Source Image: https://udemy-images.udemy.com/course/750x422/1622324_4c50.jpg

The SharePoint Framework's local webserver, used when testing your custom solutions from your development environment, uses HTTPS by default. This is implemented using a development self-signed SSL certificate. Self-signed SSL certificates are not trusted by your developer environment. You must first configure your development environment to trust the certificate.

gulp trust-dev-cert

Source: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/set-up-your-development-environment

<%@ Page Language="C#" %>
<%@ Register Tagprefix="wssawc" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %> 

The id 8be81a5c-af38-4bb2-af97-afa3b64dfbed is the Workbench app which as part of the html includes.