将文件上传到REST API的规范方法是使用multipart / form-data。正如W3推荐指南所说：

The canonical way to upload files to REST API is using the multipart/form-data. As W3 recommendation guide says:

内容类型multipart / form-data应该用于提交包含
表格文件，非ASCII数据和二进制数据。

The content type "multipart/form-data" should be used for submitting forms that contain files, non-ASCII data, and binary data.

Multipart / form-data优于base64来表示二进制数据。坚持REST / Http理念，简化API客户端的开发。

Multipart/form-data has advantages over base64 to represent binary data. Is sticked to REST/Http philosophy, and simplify the develop of API clients.

从表单返回值：multipart / form-data

W3推荐指南