Cannot set filter to custom extension

[cbossut]

As far as I understood and tested, the file filter when using Select.file(s) is set according to the mime types specified, so there is no way of setting it to a custom file extension.
Or there is a mime type that can do that that I’m don’t aware of.

[rlefevre]

Using the extension itself should be supported by most browsers, for example File.Select.file [".foo"]:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file#Unique_file_type_specifiers

See this example that allows to select a .foo file (tested in Firefox and Chrome):
https://ellie-app.com/8y27vFv7WtFa1