You have a file your computer doesn't recognise

Send an app like you send a document. That is what somebody just did, and nothing is wrong with the file: a .dai holds its own application — the thing you use it with is inside it — so your computer has nothing to open it with until you give it something.

Here is how, on whatever you are holding.

On a phone

Go to opendai.app and choose your file. That is the whole thing: a page, nothing to install, and the file never leaves your device.

Add it to your home screen and it behaves like any other app, reopening whatever you had open last — with no network, on a plane, anywhere.

On a computer

Two ways, and the first needs nothing.

Ask for it as a .dai.html instead. Whoever sent it can save it in the other form, which opens by double-click in any browser with nothing installed. Same document, same data, larger file.

Or install the desktop app. It makes .dai files behave like documents — their own icon, double-click to open, saved back into the same file rather than downloaded as a copy.

Either way it is the same file. Nothing is converted and nothing is uploaded.

What it actually is

One file holding four things that normally live in four places: an application, a real SQLite database engine, the data, and a record of what it all should be so that changes are detectable.

It cannot make a network request. Not "does not" — the browser is instructed to refuse them and does, which is why a file like this is safe to open from an email in a way that an attachment usually is not.

There is more about that, including what it does not protect you from, which is the part worth reading if you are deciding whether to trust one.

Should you open it at all?

The same question as any attachment, with one difference: this one cannot call anywhere, so it cannot report that you opened it or send anything out. What it can still do is show you things — including a convincing form asking for a password, which no format prevents.

Open files from people you know. If you want to look before you leap, the playground will read one and tell you what is inside it without running it.