fix: MediaFileUpload error if file does not exist (#1127)

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Fixes #798 🦕
diff --git a/tests/test_http.py b/tests/test_http.py
index 5293a6a..9bfae93 100644
--- a/tests/test_http.py
+++ b/tests/test_http.py
@@ -248,6 +248,10 @@
         self.assertEqual(500, new_upload.chunksize())
         self.assertEqual(b"PNG", new_upload.getbytes(1, 3))
 
+    def test_media_file_upload_raises_on_file_not_found(self):
+        with self.assertRaises(FileNotFoundError):
+            MediaFileUpload(datafile("missing.png"))
+
     def test_media_file_upload_raises_on_invalid_chunksize(self):
         self.assertRaises(
             InvalidChunkSizeError,