Add default_to_mulitpart parameter (#9335)

Co-authored-by: Yevhenii Hyzyla <hyzyla@gmail.com>
(cherry picked from commit d9a9368)

CHANGES/9335.feature.rst

@@ -0,0 +1 @@
+Added ``default_to_multipart`` parameter to ``FormData``.

aiohttp/formdata.py

@@ -23,10 +23,12 @@ def __init__(
         fields: Iterable[Any] = (),
         quote_fields: bool = True,
         charset: Optional[str] = None,
+        *,
+        default_to_multipart: bool = False,
     ) -> None:
         self._writer = multipart.MultipartWriter("form-data")
         self._fields: List[Any] = []
-        self._is_multipart = False
+        self._is_multipart = default_to_multipart
         self._is_processed = False
         self._quote_fields = quote_fields
         self._charset = charset

tests/test_formdata.py

@@ -22,14 +22,22 @@ async def write(chunk):
     return writer
 
 
-def test_formdata_multipart(buf, writer) -> None:
-    form = FormData()
+def test_formdata_multipart(buf: bytearray) -> None:
+    form = FormData(default_to_multipart=False)
     assert not form.is_multipart
 
     form.add_field("test", b"test", filename="test.txt")
     assert form.is_multipart
 
 
+def test_form_data_is_multipart_param(buf: bytearray) -> None:
+    form = FormData(default_to_multipart=True)
+    assert form.is_multipart
+
+    form.add_field("test", "test")
+    assert form.is_multipart
+
+
 def test_invalid_formdata_payload() -> None:
     form = FormData()
     form.add_field("test", object(), filename="test.txt")