Verifies that the user has permission to associate the given blob with this record.
Updating blob-valued fields is done by the user in two steps. First, the user uploads a file and
receives a unique id for the blob. Then, the user updates a record's field with the blob's unique id.
Once the blob is associated with the record, then any user with permission to view the record is extended
permission to view the blob. This opens an avenue of attack where by an attacker with seeks to obtain access
to a blob with some id by assigning it to an unrelated record to which they have access.
For this reason, only users who originally uploaded the blob may assign the blob to a record's field value.