examples: add another vweb example, showing file upload, transform, and file download (#14842)

examples/vweb/file_transform/vweb_example.v

@@ -0,0 +1,40 @@
+module main
+
+import vweb
+
+const port = 8082
+
+struct App {
+	vweb.Context
+}
+
+fn main() {
+	vweb.run(&App{}, port)
+}
+
+pub fn (mut app App) index() vweb.Result {
+	return $vweb.html()
+}
+
+['/upload'; post]
+pub fn (mut app App) upload() vweb.Result {
+	fdata := app.files['upfile']
+
+	data_rows := fdata[0].data.split('\n')
+
+	mut output_data := ''
+
+	for elem in data_rows {
+		delim_row := elem.split('\t')
+		output_data += '${delim_row[0]}\t${delim_row[1]}\t${delim_row[0].int() + delim_row[1].int()}\n'
+	}
+
+	output_data = output_data.all_before_last('\n')
+
+	println(output_data)
+
+	app.add_header('Content-Disposition', 'attachment; filename=results.txt')
+	app.send_response_to_client('application/octet-stream', output_data)
+
+	return $vweb.html()
+}