Auto merge of #25561 - pshaughn:formdata-same-file, r=jdm

Make File objects roundtrip through FormData

FormData.append now only creates a new File object if it needs to file-wrap a blob or set a filename, and not when appending an existing File with no filename override. The hardest part here was reading the specification in a way that matched browser behavior.

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- [X] These changes fix #24939

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