Fix attachment size showing 0 on the dashboard

Attachments uploaded through the SDKs showed a size of 0 and no file type on the dashboard, even though the file itself was stored and downloadable.

• Fixed the server to record each attachment's size and content type when an SDK links an uploaded file to a run
• Fixed the C#, Rust, C++ and MATLAB upload helpers to finalize uploads, and the Python client to declare file size and type up front
• Improved upload error reporting: a stored file that fails to link to its run is now reported instead of logged as success