Exclude from string results string

I would like to define a type where all the attributes are read only and looks like key:string value:string, but the 'set' attribute is a function.

I have tried the following:

type Attributes = {
  readonly [key in Exclude<string, 'set'>]:string
} & { 
  readonly set: (attr:string, value:string) => void 
}

My problem is Exclude<string, 'set'> is "simplified" back into string. Could someone with more experience explain why TypeScript does this? Some suggestions how to achive a type where I know only few property keys (but there can be many more) could help me out.

Thank you for your time and answers!

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