Add doc with info about binary upload a custom route, for #500

docs/docs/mapping/binary_file_uploads.md

@@ -0,0 +1,54 @@
+---
+layout: default
+title: Binary file uploads
+nav_order: 2
+parent: Mapping
+---
+
+# Binary file uploads
+
+If you need to do a binary file upload, e.g. via;
+
+```sh
+curl -X POST -F "attachment=@/tmp/somefile.txt" http://localhost:9090/v1/files
+```
+
+then your request will contain the binary data directly and there is no way to model this using gRPC.
+
+What you can do instead is to add a custom route directly on the `mux` instance.
+
+## Custom route on a mux instance
+
+Here we'll setup a handler (`handleBinaryFileUpload`) for `POST` requests: 
+
+```go
+// Create a mux instance
+mux := runtime.NewServeMux()
+
+// Attachment upload from http/s handled manually
+mux.HandlePath("POST", "/v1/files", handleBinaryFileUpload)
+```
+
+And then in your handler you can do something like:
+
+```go
+func handleBinaryFileUpload(w http.ResponseWriter, rq *http.Request, params map[string]string) {
+	err := r.ParseForm()
+	if err != nil {
+		http.Error(w, fmt.Sprintf("failed to parse form: %s", err.Error()), http.StatusBadRequest)
+		return
+	}
+
+	f, header, err := rq.FormFile("attachment")
+	if err != nil {
+		http.Error(w, fmt.Sprintf("failed to get file 'attachment': %s", err.Error()), http.StatusBadRequest)
+		return
+	}
+	defer f.Close()
+
+	//
+	// Now do something with the io.Reader in `f`, i.e. read it into a buffer or stream it to a gRPC client side stream.
+	// Also `header` will contain the filename, size etc of the original file.
+	//
+}
+```