Issue #4012 Updated the docs to reflect need of decoding filename

CHANGES/5831.doc

@@ -0,0 +1,4 @@
+In the current version, the filenames are encoded before sending the files to the server when using FormData() object. 
+This problem has already been posted in #4012. 
+Its fix has been discussed in #4031 will be added in aiohttp v3.8. 
+Updated the docs to help users know the problem and its fix for now.

CONTRIBUTORS.txt

@@ -285,6 +285,7 @@ Tolga Tezel
 Tomasz Trebski
 Toshiaki Tanaka
 Trinh Hoang Nhu
+Tushar Singh
 Vadim Suharnikov
 Vaibhav Sagar
 Vamsi Krishna Avula

docs/client_quickstart.rst

@@ -317,6 +317,26 @@ You can set the ``filename`` and ``content_type`` explicitly::
 
     await session.post(url, data=data)
 
+If you are sending files using ``FormData`` payload, it will automatically encode the filenames into  `urlencoding` format.
+for example, file_name and encoded_file_name will look this ::
+
+    file_name = "form file.txt"
+    encoded_file_name = "form%20file.txt"
+
+This could lead to unexpected behavior on the server side while handling filenames. To decode the encoded_file_name, we can use python's ``urllib`` module.
+following snippet shows how we can use the ``urllib`` to decode the filename::
+
+        import urllib.parse
+        encoded_file_name = "form%20file.txt"
+        decoded_file_name = urllib.parse.unquote(encoded_file_name)
+        print(decoded_file_name)
+
+Prints::
+
+        "form file.txt"
+
+In upcoming ``aiohttp v3.8``, filenames will not be encoded before sending it to the server and need of decoding will not neccessary.
+
 If you pass a file object as data parameter, aiohttp will stream it to
 the server automatically. Check :class:`~aiohttp.StreamReader`
 for supported format information.