Requires media:write. The project comes from the API key. Upload a file using multipart/form-data and use the returned Wevlix id in template components. Media is retained for 30 days. Development keys may upload without upstream delivery or charges; sending still follows normal key recipient restrictions. Limits: JPEG/PNG/WebP 5 MB; MP4/MP3 16 MB; PDF/plain text 100 MB. Template headers support JPEG/PNG, MP4 and PDF, matching the approved template.