Add ".wasm" to AllowFileExtensions and associate MimeType "application/wasm"
WebAssembly is a new binary format for static files. The official site has more info. https://webassembly.org/
Google for webassembly mimetype w3c will return many results showing the addition of this file extension and Mimetype in other mainstream servers.
wasm has been added to ServiceStack from v5.1.1 which is now available on MyGet: