Resolve "How to update appearance - upload images - via API"

doc/api/appearance.md

@@ -54,7 +54,7 @@ PUT /application/appearance
 | --------------------------------- | ------- | -------- | ----------- |
 | `title`                           | string  | no       | Instance title on the sign in / sign up page
 | `description`                     | string  | no       | Markdown text shown on the sign in / sign up page
-| `logo`                            | mixed   | no       | Instance image used on the sign in / sign up page
+| `logo`                            | mixed   | no       | Instance image used on the sign in / sign up page. See [Change logo](#change-logo)
 | `header_logo`                     | mixed   | no       | Instance image used for the main navigation bar
 | `favicon`                         | mixed   | no       | Instance favicon in `.ico` or `.png` format
 | `new_project_guidelines`          | string  | no       | Markdown text shown on the new project page
@@ -87,3 +87,36 @@ Example response:
   "email_header_and_footer_enabled": true
 }
 ```
+
+## Change logo
+
+Upload a logo to your GitLab instance.
+
+To upload an avatar from your file system, use the `--form` argument. This causes
+cURL to post data using the header `Content-Type: multipart/form-data`. The
+`file=` parameter must point to an image file on your file system and be 
+preceded by `@`.
+
+```plaintext
+PUT /application/appearance
+```
+
+| Attribute | Type   | Required | Description    |
+| --------- | ------ | -------- | -------------- |
+| `logo`    | string | Yes      | File to upload |
+
+Example request:
+
+```shell
+curl --location --request PUT "https://gitlab.example.com/api/v4/application/appearance?data=image/png" \
+--header "Content-Type: multipart/form-data" \
+--header "PRIVATE-TOKEN: <your_access_token>" \
+--form "logo=@/path/to/logo.png"
+```
+
+Returned object:
+
+```json
+{
+   "logo":"/uploads/-/system/appearance/logo/1/logo.png"
+```