I have a base64-encoded data stored in a variable, how do I send it using form-data content type?

I want to store a file as a base64-encoded variable in the collection.
I then want to use atob to convert it into a binary array and send it using form-data content type.

I’ve tried using raw tab in the postman and hand-coded the body using a variable:

Content-Disposition: form-data; name=“file”; filename=“wells.xlsx”
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet


But the binary values in the variable get encoded differently (text encoding issue maybe?)

So I try to send:
pm.variables.set(‘Test2x2AsntDefBinary’, atob(btoa(“\xFF00”)));

but the following actually being sent:
195 191 48 48

Related to this. How can I see the actual request sent?

195, 191 is a UTF8 encoding of character 255.
So when I use {{Test2x2AsntDefBinary}} in the raw body - Postman encodes it using UTF8 encoding method.

using body form-data tab and passing the same variable ({{Test2x2AsntDefBinary}}) with \xFF\xFF\xFF as content also encodes the given string using UTF8. Which is not what I want. I want to send binary in the form I give it to Postman.