Investigate and fix email subject encoding issue

CHANGELOG.md

@@ -5,6 +5,7 @@ v6.13.0
 ----------
 * Fixed from field handling in messages.send() to properly map "from_" field to "from field
 * Fixed content_id handling for large inline attachments to use content_id as field name instead of generic file{index}
+* Fixed UTF-8 character encoding for all API requests to preserve special characters (accented letters, emoji, etc.) instead of escaping them as unicode sequences
 
 v6.12.0
 ----------

examples/special_characters_demo/README.md

@@ -0,0 +1,121 @@
+# Special Characters Encoding Example
+
+This example demonstrates how the Nylas Python SDK correctly handles special characters (accented letters, unicode characters) in email subjects and message bodies.
+
+## The Problem
+
+Previously, when sending emails with large attachments (>3MB), special characters in the subject line would be incorrectly encoded. For example:
+
+- **Intended Subject:** "De l'idée à la post-prod, sans friction"
+- **What Recipients Saw:** "De l’idée à la post-prod, sans friction"
+
+This issue occurred because the SDK was using `json.dumps()` with the default `ensure_ascii=True` parameter when creating multipart/form-data requests for large attachments.
+
+## The Solution
+
+The SDK now uses `json.dumps(request_body, ensure_ascii=False)` to preserve UTF-8 characters correctly in the JSON payload, ensuring that special characters are displayed properly in recipient inboxes.
+
+## What This Example Demonstrates
+
+1. **Small Messages** - Sending messages with special characters (no attachments)
+2. **Large Messages** - Sending messages with special characters AND large attachments (>3MB)
+3. **Drafts** - Creating drafts with special characters
+4. **International Support** - Handling various international character sets
+
+## Usage
+
+### Prerequisites
+
+1. Install the SDK in development mode:
+   ```bash
+   cd /path/to/nylas-python
+   pip install -e .
+   ```
+
+2. Set up environment variables:
+   ```bash
+   export NYLAS_API_KEY="your_api_key"
+   export NYLAS_GRANT_ID="your_grant_id"
+   export RECIPIENT_EMAIL="recipient@example.com"
+   ```
+
+### Run the Example
+
+```bash
+python examples/special_characters_demo/special_characters_example.py
+```
+
+## Test Coverage
+
+This fix is covered by comprehensive tests:
+
+```bash
+# Test the core fix in file_utils
+pytest tests/utils/test_file_utils.py::TestFileUtils::test_build_form_request_with_special_characters
+
+# Test message sending with special characters
+pytest tests/resources/test_messages.py::TestMessage::test_send_message_with_special_characters_in_subject
+pytest tests/resources/test_messages.py::TestMessage::test_send_message_with_special_characters_large_attachment
+
+# Test draft creation with special characters
+pytest tests/resources/test_drafts.py::TestDraft::test_create_draft_with_special_characters_in_subject
+pytest tests/resources/test_drafts.py::TestDraft::test_create_draft_with_special_characters_large_attachment
+```
+
+## Supported Character Sets
+
+The SDK correctly handles:
+
+- **French:** é, è, ê, à, ù, ç, œ
+- **Spanish:** ñ, á, í, ó, ú, ¿, ¡
+- **German:** ä, ö, ü, ß
+- **Portuguese:** ã, õ, â, ê
+- **Italian:** à, è, é, ì, ò, ù
+- **Russian:** Cyrillic characters
+- **Japanese:** Hiragana, Katakana, Kanji
+- **Chinese:** Simplified and Traditional characters
+- **Emoji:** 🎉 🎊 🥳 and many more
+- **Special symbols:** €, £, ¥, ©, ®, ™
+
+## Technical Details
+
+### The Bug
+
+When using multipart/form-data encoding (for large attachments), the message payload was serialized as:
+
+```python
+message_payload = json.dumps(request_body)  # Default: ensure_ascii=True
+```
+
+This caused special characters to be escaped as unicode sequences:
+```json
+{"subject": "De l\u2019id\u00e9e"}
+```
+
+### The Fix
+
+The payload is now serialized as:
+
+```python
+message_payload = json.dumps(request_body, ensure_ascii=False)
+```
+
+This preserves the actual UTF-8 characters:
+```json
+{"subject": "De l'idée"}
+```
+
+The multipart/form-data Content-Type header correctly specifies UTF-8 encoding, ensuring email clients display the characters properly.
+
+## Related Files
+
+- **Core Fix:** `nylas/utils/file_utils.py` - Line 70
+- **Tests:** `tests/utils/test_file_utils.py`, `tests/resources/test_messages.py`, `tests/resources/test_drafts.py`
+- **Example:** `examples/special_characters_demo/special_characters_example.py`
+
+## Impact
+
+✅ **Before Fix:** Special characters in subjects were garbled when sending emails with large attachments  
+✅ **After Fix:** All special characters are correctly preserved and displayed
+
+The fix ensures backwards compatibility - all existing code continues to work without changes.