Insecure direct object reference

When creating an export through the pretix API, API clients are
returned an UUID value for their export job (a long, random string like
35742818-c375-4d15-839f-d49aecce94d6). Using this UUID, the API client
can then request the actual file for download. The same kind of UUID is
used in other places in pretix when temporary files are generated for
internal use or download.

One remaining API endpoint, however, wrongfully did not verify if the
UUID used for download actually belongs to a file that is supposed to
be downloadable and belongs to the correct user. In reality, this is
hard to exploit because an attacker would need to have access to a valid
UUID for the file they desire which is unlikely to happen without a
separate security problem giving them access to logs etc.