[Karan Dhaodiyal (karandhaodiyal28-hash)]

Passing files explicitly means the payload is multipart, even when no file part is present — for example Typespec-generated clients with an optional file property pass an empty files. The truthiness check in HttpRequest._set_body made that case fall into the urlencoded branch, so the request got stamped with application/x-www-form-urlencoded:

from azure.core.rest import HttpRequest

request = HttpRequest("POST", "http://example.org", data={"Hello": "world!"}, files={})
request.headers  # {'Content-Type': 'application/x-www-form-urlencoded'} — should not carry the urlencoded type

This mirrors the approach sketched by Yuchao Yan (@msyyc) in the issue: when files was explicitly passed (even empty), the urlencoded content-type must not be set, so the transport decides the multipart encoding. set_multipart_body is still only called for non-empty files, so request.files stays None in the empty case and nothing changes for existing callers.

If you'd rather have the explicit is_multipart_payload kwarg on HttpRequest as sketched in the issue, happy to add that instead.

Adds a regression test next to the existing urlencoded content-type tests.

Fixes #39163

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Thank you for your contribution Karan Dhaodiyal (@karandhaodiyal28-hash)! We will review the pull request and get back to you soon.

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[Karan Dhaodiyal (karandhaodiyal28-hash)]

The remaining test failures look unrelated to this change — they're all in dependent packages, not azure-core itself: azure-mgmt-core's httpretty-mocked tests fail with ServiceResponseError: RemoteDisconnected from the socket-mock layer, and the identity/cosmos failures are likewise in their own suites. azure-core's own tests pass. Happy to re-run if useful.