Semantically, os.path.join is the wrong choice for joining URL path components since it is OS-specific in its behaviour. This should be replaced with something that will be correct regardless when building paths for URLs.
No, that doesn't take care of strings that already have slashes in them, etc.
That lack of normalization broke the geoip stuff because the ini file setting had a slash at the end which resulted in a url like job-01//file which flask didn't like. I'd rather not end up in a situation where our config options are that persnickity again.
Fix issue #255, create function url_join
posixpath.join is the answer!