Fixing overwrite Blob/File type as Content-Type in browser. (#1773)

[MitchLewis930]

PR_027

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Note

Low Risk
Small, browser-only header-handling change that only affects requests with Blob/File bodies and should reduce incorrect Content-Type overrides.

Overview
Prevents xhrAdapter from forcing a Content-Type header when sending Blob/File request bodies that already have a type, matching the existing behavior for FormData and allowing the browser to set the correct multipart/boundary or media type.

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