File Uploads & Multipart Data ## Business Purpose

Allow sending files and form data together in a single HTTP request using multipart/form-data encoding. ## Current Behaviors - _encode_files static method in RequestEncodingMixin builds a multipart body from files and data src/requests/models.py:184. - The method processes file entries that can be 2, 3, or 4-tuples: (filename, fileobj), (filename, fileobj, content_type), (filename, fileobj, content_type, custom_headers) src/requests/models.py:184. - It raises ValueError if files is empty or if data is a string src/requests/models.py:184. - File objects with non-string filename attributes are supported src/requests/test_requests.py:1868. - Streaming upload with chunked transfer encoding is supported when body is a generator src/requests/models.py:422. ## Technical Implementation The PreparedRequest.prepare_body method calls _encode_files if files are present, otherwise uses _encode_params for standard form data. The content type header includes the boundary. ## Definition of Done - requests.post(url, files={'file': open('test.txt')}) uploads the file with correct Content-Type multipart/form-data. - A file tuple with custom content type sets that Content-Type. - Sending both data and files raises ValueError if data is a string. - A file object with a byte stream is uploaded correctly. - The boundary in the Content-Type header matches the body separator.