The browser file manager is fine for a plugin jar. For anything big, do this:
- Zip it on your computer. Right-click the folder → Compress (Windows: Send to → Compressed folder). Make sure the zip contains the folder's contents the way they should appear on the server, not an extra nesting level.
- Upload the single zip via SFTP for anything over ~100 MB, or via the browser for smaller ones.
- Unpack on the server: Files tab → right-click the zip → Unarchive. This runs on the server and takes seconds even for gigabytes.
- Delete the zip afterwards so it does not eat disk space.
Uploading from a link instead
If the file already lives somewhere on the internet (your old host's backup download, Google Drive with a direct link, a modpack server pack), skip your PC entirely: ask Kowalski "download https://... into the server root and unpack it". The server fetches it directly, so it uses no bandwidth on your side.
Disk space
Check the Files tab header or the resource bar for used space before uploading. A 2 GB zip that unpacks to 4 GB needs 6 GB free during the unpack. Upgrade disk in the dashboard if needed. It applies immediately.