dotnet/wpf · error · FileFormatException

SR.XpsValidatingLoaderThumbnailHasIncorrectType

Error message

SR.XpsValidatingLoaderThumbnailHasIncorrectType

What it means

The target of a Thumbnail relationship must be an image of type image/jpeg or image/png. When the resolved target part's Content Type matches neither, ValidateRelationships throws FileFormatException because XPS only permits JPEG and PNG thumbnails.

Solutions

1. Convert the thumbnail image to JPEG or PNG and update its part Content Type accordingly (image/jpeg or image/png).
2. Point the thumbnail relationship at an existing JPEG/PNG part.
3. Remove the thumbnail relationship if a compliant thumbnail isn't available (thumbnails are optional).
4. Check [Content_Types].xml for a mismatch between the file extension and declared Content Type.

Example fix

// before
PackagePart thumb = pkg.CreatePart(thumbUri, "image/gif");
// after
PackagePart thumb = pkg.CreatePart(thumbUri, "image/png");

foreach (var rel in fixedPart.GetRelationshipsByType(thumbnailRel)) {
    var target = package.GetPart(PackUriHelper.ResolvePartUri(fixedPart.Uri, rel.TargetUri));
    var ct = target.ContentType;
    if (!ct.Equals("image/jpeg", StringComparison.OrdinalIgnoreCase) &&
        !ct.Equals("image/png", StringComparison.OrdinalIgnoreCase))
        throw new InvalidDataException($"Thumbnail {target.Uri} must be JPEG or PNG, got {ct}");
}

static bool IsCompliantThumbnail(PackagePart p) =>
    p.ContentType.Equals("image/jpeg", StringComparison.OrdinalIgnoreCase) ||
    p.ContentType.Equals("image/png", StringComparison.OrdinalIgnoreCase);

try { loader.Load(packageStream); }
catch (FileFormatException ex) { // convert thumbnail to PNG, fix content type, retry
    ConvertThumbnailToPng(packageStream); loader.Load(packageStream); }

When it happens

Trigger: Loading an XPS package whose thumbnail relationship points to a part with Content Type image/tiff, image/gif, image/bmp, or a non-image type — detected by the AreTypeAndSubTypeEqual check against _jpgContentType/_pngContentType.

Common situations: Tools generating thumbnails in another format (BMP/GIF/TIFF) and linking them; thumbnails copied from documents that allow other image formats; parts with wrong MIME declarations in [Content_Types].xml.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• ' ' ContentType is not valid.
• ' ' ContentType is not valid.
• Document has more than one Thumbnail relationship.
• PackagePart URI does not correspond to a FixedDocument.
• PackagePart URI does not correspond to a FixedPage.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c7de054b3dd391a5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedSchema.cs:584

            count = 0;
            foreach (PackageRelationship rel in checkRels)
            {
                count++;
                if (count > 1)
                {
                    throw new FileFormatException(SR.XpsValidatingLoaderMoreThanOneThumbnailPart);
                }

                // Also check for existence and type
                Uri targetUri = PackUriHelper.ResolvePartUri(partUri, rel.TargetUri);
                Uri absTargetUri = PackUriHelper.Create(packageUri, targetUri);

                PackagePart targetPart = package.GetPart(targetUri);

                if (!_jpgContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)) &&
                    !_pngContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)))
                {
                    throw new FileFormatException(SR.XpsValidatingLoaderThumbnailHasIncorrectType);
                }
            }

            // FixedDocument only has restricted font relationships
            if (_fixedDocumentContentType.AreTypeAndSubTypeEqual(mimeType))
            {
                // Check if target of restricted font relationship is present and is actually a font
                checkRels = part.GetRelationshipsByType(_restrictedFontRel);
                foreach (PackageRelationship rel in checkRels)
                {
                    // Check for existence and type
                    Uri targetUri = PackUriHelper.ResolvePartUri(partUri, rel.TargetUri);
                    Uri absTargetUri = PackUriHelper.Create(packageUri, targetUri);

                    PackagePart targetPart = package.GetPart(targetUri);

                    if (!_fontContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)) &&
                            !_obfuscatedContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)))

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