Fixed #26651 -- Kept original file suffix in TemporaryUploadedFile name

Thanks Tim Graham for the review.

django/core/files/uploadedfile.py

@@ -58,7 +58,8 @@ class TemporaryUploadedFile(UploadedFile):
     A file uploaded to a temporary location (i.e. stream-to-disk).
     """
     def __init__(self, name, content_type, size, charset, content_type_extra=None):
-        file = tempfile.NamedTemporaryFile(suffix='.upload', dir=settings.FILE_UPLOAD_TEMP_DIR)
+        _, ext = os.path.splitext(name)
+        file = tempfile.NamedTemporaryFile(suffix='.upload' + ext, dir=settings.FILE_UPLOAD_TEMP_DIR)
         super().__init__(file, name, content_type, size, charset, content_type_extra)
 
     def temporary_file_path(self):

tests/files/tests.py

@@ -12,7 +12,8 @@ from django.core.files.base import ContentFile
 from django.core.files.move import file_move_safe
 from django.core.files.temp import NamedTemporaryFile
 from django.core.files.uploadedfile import (
-    InMemoryUploadedFile, SimpleUploadedFile, UploadedFile,
+    InMemoryUploadedFile, SimpleUploadedFile, TemporaryUploadedFile,
+    UploadedFile,
 )
 
 try:
@@ -214,6 +215,13 @@ class InMemoryUploadedFileTests(unittest.TestCase):
             self.assertEqual(f.read(), '1')
 
 
+class TemporaryUploadedFileTests(unittest.TestCase):
+    def test_extension_kept(self):
+        """The temporary file name has the same suffix as the original file."""
+        with TemporaryUploadedFile('test.txt', 'text/plain', 1, 'utf8') as temp_file:
+            self.assertTrue(temp_file.file.name.endswith('.upload.txt'))
+
+
 class DimensionClosingBug(unittest.TestCase):
     """
     get_image_dimensions() properly closes files (#8817)