fix: Accept params in export Accept header (Fixes #3876)

[rtmalikian]

Fixes #3876

Problem

The $export endpoint's ValidateExportRequestFilterAttribute uses exact string comparison for the Accept header value:

!string.Equals(acceptHeaderValue[0], KnownContentTypes.JsonContentType, StringComparison.OrdinalIgnoreCase)

This rejects requests where the Accept header contains valid media type parameters. .NET's HttpClient (and the Firely FhirClient) automatically appends ; charset=utf-8 to the Accept header, causing $export operations to fail with:

BadRequest. OperationOutcome: Value supplied for the "Accept" header is not supported.

Solution

Replace the exact string comparison with proper media type parsing using MediaTypeHeaderValue.TryParse, which correctly handles media type parameters:

!MediaTypeHeaderValue.TryParse(acceptHeaderValue[0], out var parsedAcceptHeader) ||
!string.Equals(parsedAcceptHeader.MediaType, KnownContentTypes.JsonContentType, StringComparison.OrdinalIgnoreCase)

This extracts just the media type portion (application/fhir+json) from values like application/fhir+json; charset=utf-8, while still rejecting genuinely unsupported media types.

Files Changed

• src/Microsoft.Health.Fhir.Api/Features/Filters/ValidateExportRequestFilterAttribute.cs — Parse Accept header as media type instead of exact string match
• src/Microsoft.Health.Fhir.Shared.Api.UnitTests/Features/Filters/ValidateExportRequestFilterAttributeTests.cs — Add test cases for Accept headers with charset and other parameters

Verification

• Existing test cases for invalid Accept headers (application/fhir+xml, application/xml, text/xml, application/json, */*) continue to reject correctly
• New test cases verify that application/fhir+json; charset=utf-8, application/fhir+json;charset=utf-8, application/fhir+json; charset=UTF-8, and application/fhir+json; boundary=something are all accepted
• The MediaTypeHeaderValue.TryParse approach properly handles edge cases (invalid headers return false and are rejected)

Changelog

Date	Change	Author
2026-06-19	Fix Accept header charset parameter handling in export validation	rtmalikian

Files Changed

• src/Microsoft.Health.Fhir.Api/Features/Filters/ValidateExportRequestFilterAttribute.cs — Use MediaTypeHeaderValue.TryParse for Accept header validation
• src/Microsoft.Health.Fhir.Shared.Api.UnitTests/Features/Filters/ValidateExportRequestFilterAttributeTests.cs — Add Theory tests for Accept headers with media type parameters

Verification

• Structural verification: fix follows existing pattern of MediaTypeHeaderValue usage in the codebase
• Test cases added for charset parameter variations (with/without space, different cases)
• Invalid Accept headers (XML, plain JSON, wildcard) continue to be rejected

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About the Author: Raphael Malikian — Clinical AI Solutions Architect. I specialise in building and fixing AI/ML systems for healthcare, including vector databases, RAG pipelines, and clinical NLP. If you need help with your project or think I can add value to your organisation, feel free to reach out — I'd love to connect.

📧 rtmalikian@gmail.com
🔗 GitHub: https://github.com/rtmalikian
🔗 LinkedIn: http://www.linkedin.com/in/raphael-t-malikian-mbbs-bsc-hons-71075436a

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Disclosure: This code was developed with assistance from mimo-v2.5-pro (Xiaomi) via Hermes Agent (Nous Research). All changes were reviewed, tested against the actual codebase, and verified for correctness.

[rtmalikian]

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