Extending lib.d.ts interfaces? - intellij-idea

Question

I have some code that handles a drop event and checks if I have to deal with dropped files:
public handleDrop(evt: DropEvent): boolean {
var isUpload: boolean; = false;
var dt: DataTransfer = evt.DataTransfer;
dt.types.forEach((type)=>{
// dt.types is of type DOMStringList
if(type === "File"){
isUpload = true;
}
})
return isUpload;
}
It works just as it is supposed to be, but the TS-compiler gives me following error message:
(117,16): error TS2339: Property 'forEach' does not exist on type 'DOMStringList'.
First of all, DataTransfer.types will always be an array, so in my understanding I should have access to its array methods.
I tried to create a workaround for my problem, by merging a custom DOMStringList Interface to the one defined in my lib.d.ts:
declare interface DOMStringList {
forEach(cb: Function): any;
}
IntelliJ recognizes the interface and does not show me an error for dt.types.forEach anymore.
However my ts-compiler still throws an error and does not seem to recognize my merged interface.
What is the proper way to tell my ts-compiler that I should be able to use array functions on an DOMStringList?
The problem occurs for other generic types like FileList as well.
EDIT:
ts.config.ts:
{
"compilerOptions": {
"target": "ES5",
"module": "commonjs",
"noEmitOnError": false,
"noImplicitAny": false,
"removeComments": true,
"preserveConstEnums": true,
"sourceMap": true
},
"files": [
"app/typings/app.typings.ts",
"typings/tsd.d.ts"
]
}