Invalid Extension

$$404 \in \begin{cases}100& \text{if Continue: Request received, please continue}\\101& \text{if Switching Protocols: Switching to new protocol; obey Upgrade header}\\200& \text{if OK: Request fulfilled, document follows}\\201& \text{if Created: Document created, URL follows}\\202& \text{if Accepted: Request accepted, processing continues off-line}\\203& \text{if Non-Authoritative Information: Request fulfilled from cache}\\204& \text{if No Content: Request fulfilled, nothing follows}\\205& \text{if Reset Content: Clear input form for further input.}\\206& \text{if Partial Content: Partial content follows.}\\300& \text{if Multiple Choices: Object has several resources -- see URI list}\\301& \text{if Moved Permanently: Object moved permanently -- see URI list}\\302& \text{if Found: Object moved temporarily -- see URI list}\\303& \text{if See Other: Object moved -- see Method and URL list}\\304& \text{if Not Modified: Document has not changed since given time}\\305& \text{if Use Proxy: You must use proxy specified in Location to access this resource.}\\307& \text{if Temporary Redirect: Object moved temporarily -- see URI list}\\400& \text{if Bad Request: Bad request syntax or unsupported method}\\401& \text{if Unauthorized: No permission -- see authorization schemes}\\402& \text{if Payment Required: No payment -- see charging schemes}\\403& \text{if Forbidden: Request forbidden -- authorization will not help}\\404& \text{if Not Found: Nothing matches the given URI}\\405& \text{if Method Not Allowed: Specified method is invalid for this server.}\\406& \text{if Not Acceptable: URI not available in preferred format.}\\407& \text{if Proxy Authentication Required: You must authenticate with this proxy before proceeding.}\\408& \text{if Request Timeout: Request timed out; try again later.}\\409& \text{if Conflict: Request conflict.}\\410& \text{if Gone: URI no longer exists and has been permanently removed.}\\411& \text{if Length Required: Client must specify Content-Length.}\\412& \text{if Precondition Failed: Precondition in headers is false.}\\413& \text{if Request Entity Too Large: Entity is too large.}\\414& \text{if Request-URI Too Long: URI is too long.}\\415& \text{if Unsupported Media Type: Entity body in unsupported format.}\\416& \text{if Requested Range Not Satisfiable: Cannot satisfy request range.}\\417& \text{if Expectation Failed: Expect condition could not be satisfied.}\\500& \text{if Internal Server Error: Server got itself in trouble}\\501& \text{if Not Implemented: Server does not support this operation}\\502& \text{if Bad Gateway: Invalid responses from another server/proxy.}\\503& \text{if Service Unavailable: The server cannot process the request due to a high load}\\504& \text{if Gateway Timeout: The gateway server did not receive a timely response}\\505& \text{if HTTP Version Not Supported: Cannot fulfill request.}\\\end{cases}$$