Define what File Explorer should do
Double-clicking a file and adding it to an existing queue are different tasks. Begin with the intended behavior: open this file now or append it behind the current item. Keep the current list saved if it matters and use two harmless local samples for the first comparison.
Record the actual executable launched by the file action. A stale Windows association can start another copy or another player, making a correct queue setting appear ineffective. Keep application identity and queue behavior as separate checkpoints.
Use the documented instance condition
The mpv.net manual documents --queue for playlist addition with process-instance set to single. It also describes supported Explorer integration routes. Inspect the installed build’s current instance configuration and the actual command used by the chosen integration.
Do not copy a shell command containing a private media URL into a public report. For the first test, use an ordinary local path with spaces handled by the actual launching tool. Keep the command and input type recorded privately so an argument-parsing problem can be distinguished from a queue issue.
Compare the receiving session explicitly
Leave the first sample playing and send the second through the intended append action. Inspect the same player’s queue and verify where the new item appears. A new window playing the second file is a different result from the existing session receiving it.
Advance through the queue deliberately and check that both references open. If the second item is present but inaccessible, preserve that later-stage source result. Do not re-register every Windows association while the first question is whether the intended process received the argument.
Check the ordinary desktop workflow
After a successful small test, repeat through the actual Explorer control used in everyday work. Keep any third-party shell-extension identity with the notes because it is another dependency. A direct terminal test and an Explorer handoff can construct arguments differently.
The expected outcome is predictable appending to the intended instance. It does not copy media into storage, make every browser URL playable or guarantee the same behavior in standalone mpv. Preserve the active instance, integration route and observed list for support.
Questions answered
Does opening another player window prove append succeeded?
No. Inspect the existing session’s queue and actual receiving executable.
Why record the Explorer integration separately from a terminal test?
The handoff can construct arguments differently and may depend on another shell component.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.