I have problem with the same relation path, the test case not able to run in Windows and Linux with the same path.
So the solution here:
import java.nio.file.Path as Path
import java.nio.file.Paths as Paths
String filePath = RunConfiguration.getProjectDir() + '/Data Files/test-doc.pdf'
Path absolutePath = Paths.get(filePath)
String absolutePathStr = absolutePath.toString()
WebUI.uploadFile(findTestObject('object'), absolutePathStr)