docs: add doc for upload file

docs/prepare/upload-case.md

@@ -0,0 +1,51 @@
+
+对于上传文件类型的测试场景，HttpRunner 集成 [requests_toolbelt][1] 实现了上传功能。
+
+在使用之前，确保已安装如下依赖库：
+
+- [requests_toolbelt](https://github.com/requests/toolbelt)
+- [filetype](https://github.com/h2non/filetype.py)
+
+使用内置 `upload` 关键字，可轻松实现上传功能。（适用版本：2.4.1+）
+
+```yaml
+- test:
+    name: upload file
+    request:
+        url: http://httpbin.org/upload
+        method: POST
+        headers:
+            Cookie: session=AAA-BBB-CCC
+        upload:
+            file: "data/file_to_upload"
+            field1: "value1"
+            field2: "value2"
+    validate:
+        - eq: ["status_code", 200]
+```
+
+同时，你也可以继续使用之前描述形式。（使用版本：2.0+）
+
+```yaml
+- test:
+    name: upload file
+    variables:
+        file: "data/file_to_upload"
+        field1: "value1"
+        field2: "value2"
+        m_encoder: ${multipart_encoder(file=$file, field1=$field1, field2=$field2)}
+    request:
+        url: http://httpbin.org/upload
+        method: POST
+        headers:
+            Content-Type: ${multipart_content_type($m_encoder)}
+            Cookie: session=AAA-BBB-CCC
+        data: $m_encoder
+    validate:
+        - eq: ["status_code", 200]
+```
+
+参考案例：[httprunner/tests/httpbin/upload.v2.yml][2]
+
+[1]: https://toolbelt.readthedocs.io/en/latest/uploading-data.html
+[2]: https://github.com/httprunner/httprunner/blob/master/tests/httpbin/upload.v2.yml