FileURLConnection uses URLDecoder.decode() as part of its process for turning URL paths into filesystem paths. However, this causes paths with '' symbols are turned into spaces. (This is most relevant on mac os x, where the temp directory often contains plus symbols).
This bug is basically identical to
JBVFS-129, and I believe the fix is the same (use File's constructor taking a URI argument).
I'll attach a patch with a testcase and a patch with a potential fix.