docs: add a file upload section to admin (#1227)

Author: alanpoulain

admin/file-upload.md

@@ -0,0 +1,37 @@
+# Handling File Upload
+
+If you need to handle the file upload in the server part, please follow [the related documentation](../core/file-upload.md).
+
+This documentation assumes you have a `/media_objects` endpoint accepting `multipart/form-data`-encoded data.
+
+To manage the upload in the admin part, you need to customize [the create form](customizing.md#customizing-the-create-form) or [the edit form](customizing.md#customizing-the-edit-form).
+
+Add a [FileInput](https://marmelab.com/react-admin/Inputs.html#fileinput) as a child of the guesser. For example, for the create form:
+
+```js
+import {
+  HydraAdmin,
+  ResourceGuesser,
+  CreateGuesser,
+  InputGuesser
+} from "@api-platform/admin";
+import { FileField, FileInput } from "react-admin";
+
+const MediaObjectsCreate = props => (
+  <CreateGuesser {...props}>
+    <FileInput source="file">
+      <FileField source="src" title="title" />
+    </FileInput>
+  </CreateGuesser>
+);
+
+export default () => (
+  <HydraAdmin entrypoint="https://demo.api-platform.com">
+    <ResourceGuesser name="media_objects" create={MediaObjectsCreate} />
+    {/* ... */}
+  </HydraAdmin>
+);
+```
+
+And that's it!
+You don't need to decorate the data provider if you are using the Hydra one: it detects that you have used a `FileInput` and uses a `multipart/form-data` request instead of a JSON-LD one.

outline.yaml

@@ -64,6 +64,7 @@ chapters:
       - handling-relations
       - schema.org
       - authentication-support
+      - file-upload
       - performance
       - customizing
       - components