Usually uploading of HTML files from a third party application you usually don't want done because of security risks.
With that said, are you talking about the file manager within plesk or is this an web application that you've created or created by someone else? If it's a web application that was created by someone else, it's possible that they've set a hard limit of acceptable file types. If it's a web application you've created you'll want to check that you didn't set a hard limit for it.
Since you weren't specific with what kind of web application it was or what it's written in, I could only provide you with generic guidance.