Add accessor function to File member of basic_file_body:

fix #1260
close #1814

include/boost/beast/http/basic_file_body.hpp

@@ -111,6 +111,12 @@ public:
     /// Move assignment
     value_type& operator=(value_type&& other) = default;
 
+    /// Return the file
+    File& file()
+    {
+        return file_;
+    }
+
     /// Returns `true` if the file is open
     bool
     is_open() const

include/boost/beast/http/impl/file_body_win32.hpp

@@ -78,6 +78,11 @@ struct basic_file_body<file_win32>
         value_type(value_type&& other) = default;
         value_type& operator=(value_type&& other) = default;
 
+        file_win32& file()
+        {
+            return file_;
+        }
+
         bool
         is_open() const
         {