fails on prefill in ie11

[kroegen]

ie11 doesn't support File constructor OOB
I fixed this with next code found here jimmywarting/FormData#11 :
let File = window.File

try {
new File([], '')
} catch(e) {
File = class File extends Blob {
constructor(chunks, filename, opts = {}){
super(chunks, opts)
this.lastModifiedDate = new Date()
this.lastModified =+ this.lastModifiedDate
this.name = filename
}
}
}

[alessiomaffeis]

Great, thanks for the heads-up! Would you mind to create a PR? :)