Raise a more obvious exception when form data is missing "boundary".

starlette/formparsers.py

@@ -159,7 +159,7 @@ async def parse(self) -> FormData:
         charset = params.get(b"charset", "utf-8")
         if type(charset) == bytes:
             charset = charset.decode("latin-1")
-        boundary = params.get(b"boundary")
+        boundary = params[b"boundary"]
 
         # Callbacks dictionary.
         callbacks = {

tests/test_formparsers.py

@@ -1,6 +1,8 @@
 import os
 import typing
 
+import pytest
+
 from starlette.formparsers import UploadFile, _user_safe_decode
 from starlette.requests import Request
 from starlette.responses import JSONResponse
@@ -386,3 +388,18 @@ def test_user_safe_decode_helper():
 def test_user_safe_decode_ignores_wrong_charset():
     result = _user_safe_decode(b"abc", "latin-8")
     assert result == "abc"
+
+
+def test_missing_boundary_parameter(test_client_factory):
+    client = test_client_factory(app)
+    with pytest.raises(KeyError, match="boundary"):
+        client.post(
+            "/",
+            data=(
+                # file
+                b'Content-Disposition: form-data; name="file"; filename="\xe6\x96\x87\xe6\x9b\xb8.txt"\r\n'  # noqa: E501
+                b"Content-Type: text/plain\r\n\r\n"
+                b"<file content>\r\n"
+            ),
+            headers={"Content-Type": "multipart/form-data; charset=utf-8"},
+        )