File pick + upload seam. Binary file bytes can't safely cross the core<->platform boundary on web (Embind string marshaling) or Android (jstring/modified-UTF-8), so rather than route an upload's bytes through http(), the platform owns the whole operation: it opens a native file picker (filtered by accept, an HTML- style MIME list like "image/png,image/jpeg"), then multipart-uploads the chosen file — under form field fieldName, with the HTTP verb method (e.g. "POST" for post attachments, "PUT" for a profile picture) — to url with headers (e.g. an Authorization bearer), and delivers the API's JSON reply back as a string. The binary never enters the core; only the response text does. Per-platform shells inject the transport at startup (desktop QFileDialog+QHttpMultiPart, web +FormData/fetch, Android SAF+OkHttp). Backs attachment uploads (POST {profile}/api/uploads) and the profile picture (PUT {profile}/api/profile/ picture). [Attachments]