fup: fix minor issue with shorter-than-512 byte file uploads

Reader.Read is permitted to return EOF on short reads, which had not been
anticipated by this code. There's probably more instances of this lurking...

web/fup/fuphttp/httpupload.go

@@ -174,7 +174,7 @@ func (a *Application) upload(rw http.ResponseWriter, r *http.Request) {
 	if mimeType == "" {
 		// We'll need to sniff it...
 		buf := make([]byte, 512)
-		if _, err := r.Body.Read(buf); err != nil {
+		if _, err := r.Body.Read(buf); err != nil && !errors.Is(err, io.EOF) {
 			log.Printf("upload: Read for MIME sniffing: %v", err)
 			a.internalError(rw, r)
 			return