createCollection/createIndexes accept storageEngine.wiredTiger.configString, but the validator only blocked NUL bytes, the literal substring type=lsm, and encryption options. A quoted type="file" passed validation, overrode the server's bare type=file (WiredTiger last-wins), and was stored in durable metadata as a string token instead of an identifier token. Any later collStats/validate read that metadata back and tripped an invariant in WiredTigerUtil::fetchTypeAndSourceURI, aborting mongod. The poisoned metadata is durable and replicated, so this is a persistent DoS.

This change rejects any user-supplied type in configString except a redundant, bare type=file, closing the validation gap. It also covers the quoted type="lsm" variant that bypassed the existing substring check.