修复重构loader后，http上传失败的问题

Aria/src/main/java/com/arialyy/aria/core/common/HttpOption.java

@@ -35,6 +35,7 @@ public class HttpOption extends BaseOption {
   private Proxy proxy;
   private boolean useServerFileName = false;
   private IHttpFileLenAdapter fileLenAdapter;
+  private String attachment;
 
   public HttpOption() {
     super();
@@ -84,6 +85,19 @@ public class HttpOption extends BaseOption {
     return this;
   }
 
+  /**
+   * 设置文件上传需要的key
+   *
+   * @param attachment 如果为空，默认为"file"
+   */
+  public HttpOption setAttachment(String attachment) {
+    if (TextUtils.isEmpty(attachment)) {
+      attachment = "file";
+    }
+    this.attachment = attachment;
+    return this;
+  }
+
   /**
    * 给url请求添加Header数据
    * 如果新的header数据和数据保存的不一致，则更新数据库中对应的header数据