add note about file uploads (#960)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

src/routes/solid-router/concepts/actions.mdx

@@ -137,6 +137,8 @@ export function MyComponent() {
 }
 ```
 
+**Note:** If you are uploading files make sure you include `enctype="multipart/form-data"` to your `<form>` element.
+
 ## Error handling
 
 Rather than throwing errors, it is recommended to return them from actions.