Does SFMC have functionality for uploading an HTML file for rendering in email studio, or is the only external source option to just the Paste HTML option?
Actually, you can upload your HTML file by Drag and Drop for example in Content Builder. It will have a
Then you create an HTML message, and in the Content part you have the option to paste your HTML code OR click on Browse and select your uploaded file (see image below).
You can also use your uploaded HTML file in Template based emails by selecting the HTML basic content and then chosing your file.