`wrangler preview` fails with "module with name '_headers' must have content type 'text/plain'"

Finding: In an external report on cloudflare/workers-sdk, `wrangler preview` reportedly fails after a successful asset upload with Cloudflare API error 10021, 'module with name _headers must have content type text/plain'. The report says this happens for SvelteKit apps using adapter-cloudflare that have both a Worker `main` script and a root `_headers` file. Source: https://github.com/cloudflare/workers-sdk/issues/15791. Environment given by the reporter: macOS 27.0 arm64 with Node 26.9.0, and a GitHub Actions Linux runner. The exact Wrangler version is not stated; a later comment says 'latest'. Reported pattern (external report, per the issue body): five apps with both a Worker script and `_headers` fail every time, including on a freshly created Preview. One assets-only app with `_headers` and no `main` succeeds. One hand-written Worker without `_headers` succeeds. The reporter also says `wrangler deploy` and `wrangler versions upload` succeed for the same build output. The failure occurs in the separate `/previews/<id>/deployments` request, after the log line 'Uploaded N files'. Status: A repository MEMBER said they tried to reproduce and the upload worked, noted no recent Wrangler-side changes, and suggested an upstream change by the preview team (https://github.com/cloudflare/workers-sdk/issues/15791#issuecomment-6018180964). The reporter replied on 2026-10-08 that the failure still occurs on latest Wrangler, with the same error and code 10021 (https://github.com/cloudflare/workers-sdk/issues/15791#issuecomment-6067910009). An automated triage comment, generated by an LLM, labeled it actionable but is not independent evidence (https://github.com/cloudflare/workers-sdk/issues/15791#issuecomment-5784086737). Cause: unconfirmed. The idea that the preview deployment treats `_headers` as a Worker module is the reporter's hypothesis. The maintainer comment does not confirm it. Diagnostic hint (limited): if you see this exact error, check whether the project combines a Worker `main` with a root `_headers` file, and whether `wrangler deploy` succeeds on the same output. That matches the reported differential only. Limitations: this is a single-reporter external report, with five apps in one monorepo. One maintainer could not reproduce it. No minimal reproduction, no exact Wrangler version, no fix commit and no released fix appear in the supplied sources. This record was not reproduced by KFA.