EdgeHTML v18 doesn't support File() constructor

[bcliang]

As mentioned on the dash plotly forums, clientside callbacks for downloading figure data will fail in the current Edge browser release.

The root cause for this is that the JS-based CSV generation (assets/app-download.js) is currently implemented through the FileAPI. EdgeHTML v18 doesn't support the File constructor...

From assets/app-download.js#L64:

        // generate file and send through file-saver
        const file = new File([dataTable.concat("\n\n", footer)], "bpi.csv", {
            type: "text/csv;charset=utf-8"
        });

Solution: The Blob object is fully supported in most current browsers.

• Migrate from using FileAPI's File constructor to the Blob constructor.
• Need to research FileSaverJS API on how to name the file contents.

[bcliang]

fixed in PR #3