Before uploading...

Download the EU4SaveTrimmer (Yes, it's an exe. If someone knows how to make a decent batch file or something... I'm open to suggestions).
Backup your EU4 save, then drag/drop it on the exe on your computer. It will take a couple minutes and spit out a .eu4.trim file.

Source is here if you feel paranoid and have a C# compiler.

It doesn't matter how fast your bandwidth is, this server connection can't handle multi-MB uploads, so TRIM YOUR SAVE.