Allow `showOpenDialog` to open local files on web

In the demo in https://github.com/microsoft/vscode-internalbacklog/issues/2178#issuecomment-901514989, I had to manually create a file in the web workspace since I couldn’t figure out a way to get vscode.window.showOpenDialog to open a file on the user’s machine. Rob suggested that adding a file:/// URI as the defaultUri might work, but it doesn’t.

Is opening a local file in this way already possible, and if not could we add a way for it to happen, perhaps using the defaultUri as Rob suggested?

[connor4312]

Sounds good. Let’s dive into this more next iteration; Kai would like to get the referenced experience going by the end of this year 😃

[alexr00]

There was some discussion somewhere about whether we should allow extensions to open local files on web which I now cannot find. If I remember correctly, the outcome of the discussion was that we wouldn’t allow it for now because:

• we didn’t have compelling reason to allow it.
• it was probably almost never what the extension wanted to do and more likely would result from a programming error.

Now we have a real need for it now though. Given the potential for error here, we should consider making the local file option for vscode.window.showOpenDialog opt-in, and not something that could be done by accident. Similar to the availableFilesystems option we have internally:

https://github.com/microsoft/vscode/blob/ee1655a82ebdfd38bf8792088a6602c69f7bbd94/src/vs/platform/dialogs/common/dialogs.ts#L101-L107