Improving documentation on file uploads

Adding information about setting multipart/form-data on forms in order
to use file uploads.

laravel/documentation/input.md

@@ -69,6 +69,12 @@ #### Retrieving a specific item from a $_FILES array:
 
 	$size = Input::file('picture.size');
 
+> **Note:** In order to use file uploads, you must use `Form::open_for_files()` or manually enable `multipart/form-data`.
+
+*Further Reading:*
+
+- *[Openning Forms](/docs/views/forms#opening-a-form)*
+
 <a name="old-input"></a>
 ## Old Input