There's more to it. If a file is set to download rather than play, it may work in the iTunes application but it probably won't work in the Store. The test is that if you paste the URL into Safari it plays in Safari in an otherwise blank page, not in a player, and doesn't download.
But in any case evidently DropBox doesn't handle ' byte range requests', which are necessary for iPhones: the Store checks for this and will not accept podcasts whose media files are not hosted on a service with this facility.
Avoid Dropbox; it's excellent for file storage and syncing; it's not suitable for podcasting.