can't open downloaded file

I'm trying to download a file in my flutter app using the Dio plugin.

The downloaded file appears in the download folder on my phone but I can't open it.
it doesn't have a mime type.

It's the same for every mime type (jpg, pdf, etc...)

[how the downloaded file looks in the download folder]
https://i.stack.imgur.com/JRhmR.png

Can anyone help?