Use the browser as the conversion boundary. Choose a local MIDI file, review the standard and extended notation options, and generate the sheet in place. That keeps the workflow concrete: file in, notation out, and no server upload.
Once the notation is ready, scan it for the phrases and timings that matter to your Roblox practice session. Clean transitions and stable note runs matter more than raw density. That makes the sheet usable for learning inside a Roblox piano workflow instead of implying a dedicated Roblox format.
If you still need a file source, go back to the download guide and pull from the accessible walkthrough there before returning to the converter and player.
Convert, then practice
Use these steps in order with a local MIDI file such as the sample shown below.

