Press ALT > Tools > Folder Options
View Tab > Don't check "Hide extension for known file types"
then see the extension. the extension is located after filename(e.x.: de_city.zip) if zip, please contact DC. else, try to change the exension(not rename him, that will lead to problems) and try to upload it
make sure that it is really a ZIP file and not a RAR file. winrar can create zip files as well, but you have to select zip. rar is default there. real zip files should work fine. other compression formats like rar or gzip or whatever do NOT work! zip only!