downloaded template to desktop, edited (in notepad+), saved with a new name to desktop, created new customisation entry and browse to desktop to file...upload takes 1 or 2 seconds and shows me a file size in bytes. Click OK. [The import button just sends you back to choose another file.]. Done. Preview always a good idea.
A couple of times the edited .htm didn't upload because I timed out on the main ZD login page. I never saw your particular fail.
User permissions - wild stab. Do YOU have permission to access MMIAH as per screenshot?
File not saved as a webpage (htm for instance).
File too big (put a picture in possibly?).