I just uploaded a Mac Numbers file to Dropbox by dragging and dropping from my Mac desktop to Dropbox.com in Chrome. The file become a folder with 7 files in it in Dropbox. Why did this happen?
Because iWork "files" aren't actually files, they're packages. And a package in OS X is a folder with some special flag that lets OS X recognise it as a package. Other systems (e.g. DropBox) don't recognise this and just treat it as a folder. To see what's inside a package in OS X, right-click on the package in Finder and select "Show Package Contents".