shoc-backend/SeaHaven.Services/Helpers/WorkOrderCompletionDocFileRules.cs

using Microsoft.AspNetCore.Http;

namespace SeaHaven.Services.Helpers
{
    /// <summary>SH-337 file type allowlist for work-order completion documents.</summary>
    public static class WorkOrderCompletionDocFileRules
    {
        private const string PdfContentType = "application/pdf";

        private static readonly HashSet<string> AllowedExtensions =
            new(StringComparer.OrdinalIgnoreCase) { ".pdf" };

        // A browser sets the multipart part's Content-Type from File.type, which the OS
        // sometimes leaves empty for a PDF. The completion-doc dialog already accepts
        // that case on the client (a .pdf name with no type passes), so rejecting it
        // here would fail uploads that work today. The %PDF- signature is the real gate.
        private static readonly HashSet<string> UndeterminedContentTypes =
            new(StringComparer.OrdinalIgnoreCase) { "application/octet-stream" };

        public static bool IsAllowed(IFormFile file)
        {
            if (file == null || file.Length <= 0)
                return false;

            var declaredType = (file.ContentType ?? string.Empty).Trim();
            if (!string.IsNullOrWhiteSpace(declaredType)
                && !declaredType.Equals(PdfContentType, StringComparison.OrdinalIgnoreCase)
                && !UndeterminedContentTypes.Contains(declaredType))
            {
                return false;
            }

            var extension = Path.GetExtension(file.FileName ?? string.Empty);
            if (string.IsNullOrWhiteSpace(extension) || !AllowedExtensions.Contains(extension))
                return false;

            try
            {
                using var stream = file.OpenReadStream();
                var headerLength = (int)Math.Min(Math.Max(file.Length, 0), 64);
                if (headerLength == 0)
                    return false;

                var header = new byte[headerLength];
                var read = stream.Read(header, 0, header.Length);
                if (read <= 0)
                    return false;

                if (read < header.Length)
                    Array.Resize(ref header, read);

                return WorkOrderMediaFileRules.MatchesSignature(PdfContentType, header);
            }
            catch
            {
                return false;
            }
        }

        public static void EnsureAllowed(IFormFile file)
        {
            if (!IsAllowed(file))
            {
                throw new Exceptions.WorkOrderBoardValidationException(
                    "UnsupportedMediaType",
                    "The completion document must be a PDF file.");
            }
        }
    }
}