Storage API: Case-sensitive "Content-Type" header causes multipart boundary storage instead of file content

Supabase storage API stores multipart form data boundaries instead of actual file content when using capitalized "Content-Type" header. Files upload successfully (200 OK) but contain the entire multipart envelope rather than just the file bytes. Upload file using capitalized header: ```python # ❌ This causes the issue headers = {"Content-Type": file.content_type, "upsert": "true"} ``` File gets stored with multipart boundaries: ``` --boundary123\r\nContent-Disposition: form-data; name="file"; filename="test.pdf"\r\nContent-Type: application/pdf\r\n\r\n%PDF-1.4... ``` Downloads return malformed content instead of actual file **Expected behavior** File should be stored with just the file content, regardless of header capitalization. **Actual behavior** - Capitalized "Content-Type": Stores multipart boundaries ❌ - Lowercase "content-type": Stores file content correctly ✅ **Environment** Supabase Python client: v2.17.0 Environment: Local development Python version: 3.12.11 **Additional context** - **_HTTP RFC states headers should be case-insensitive_** - This affects both Python client and direct REST API calls - Related to multipart form parsing in storage backend - No error thrown - files appear to upload successfully but contain wrong data **Minimal Replication** ```python from fastapi import UploadFile from supabase import create_client # This breaks result = supabase.storage.from_("bucket").upload( filename, file_content, file_options={"Content-Type": file.content_type} # ❌ ) # This works result = supabase.storage.from_("bucket").upload( filename, file_content, file_options={"content-type": file.content_type} # ✅ ) ```