Add notes on "filename" form-data field

docs/requests.md

@@ -112,7 +112,15 @@ state with `disconnected = await request.is_disconnected()`.
 
 #### Request Files
 
-Request files are normally sent as multipart form data (`multipart/form-data`).
+Request files are normally sent as multipart form data (`multipart/form-data`). 
+
+!!! note
+    As settled in [RFC-7578: 4.2](https://www.ietf.org/rfc/rfc7578.txt), form-data content part that contains file 
+    assumed to have `name` and `filename` fields in `Content-Disposition` header: `Content-Disposition: form-
+    data; name="user"; filename="somefile"`. Though `filename` field is optional according to RFC-7578, it helps 
+    Starlette to differentiate which data should be treated as file. If `filename` field was supplied, `UploadFile` 
+    object will be created to access underlying file, otherwise form-data part will be parsed and available as a raw 
+    string.
 
 When you call `await request.form()` you receive a `starlette.datastructures.FormData` which is an immutable
 multidict, containing both file uploads and text input. File upload items are represented as instances of `starlette.datastructures.UploadFile`.
@@ -138,7 +146,7 @@ For example, you can get the file name and the contents with:
 
 ```python
 form = await request.form()
-filename = form["upload_file"].filename
+filename = form["upload_file"].filename # or form.get("upload_file")
 contents = await form["upload_file"].read()
 ```