One media file a saved document refers to.
The asset's id inside the document.
Size plus SHA-256 of the first and last 64 KB ("<bytes>:<hex>"): cheap, and the same key the editor uses to know a picked file and its stored copy are one file.
"<bytes>:<hex>"
The file name the user imported.
A handle to the stored file, not its bytes; read it as a stream or upload it.
One media file a saved document refers to.