Uploading local dataset in Firefox ESR is broken (windows only)

[hudson-newey]

Providing a local dataset through Windows Firefox ESR (version 128.4.0esr 64-bit) doens't work

It fails with the error message "Unsupported file format"

I suspect this is something to do with Firefox converting the file into a blob without a mime type. So that when we use the file constructor, it doesn't have a file.type