iOfficeAI/OfficeCLI · error · ArgumentException

Unsupported MIME type: {mime}. Supported: image/png, image/j

Error message

Unsupported MIME type: {mime}. Supported: image/png, image/jpeg, image/gif, image/bmp, image/tiff, image/svg+xml

What it means

Thrown by ImageSource.MimeToContentType (a private helper called from ResolveDataUri and indirectly from ResolveUrl) when a MIME type string doesn't match any of the supported image MIME types. The supported set is: image/png, image/jpeg (or image/jpg), image/gif, image/bmp, image/tiff (or image/tif), image/svg+xml, image/emf (or image/x-emf), image/wmf (or image/x-wmf). Any other MIME type triggers this error. Unlike TryMimeToContentType which returns false gracefully, MimeToContentType throws.

Source

Thrown at src/officecli/Core/ImageSource.cs:189

            return (stream, ct);

        // Fallback: extract extension from URL path (strip query string)
        var uri = new Uri(url);
        var ext = Path.GetExtension(uri.AbsolutePath);
        if (!string.IsNullOrEmpty(ext))
            return (stream, ExtensionToContentType(ext));

        // Last resort: sniff magic bytes
        if (TrySniffContentType(bytes, out var sniffed))
            return (stream, sniffed);

        throw new ArgumentException($"Cannot determine image type from URL: {url}. Specify format via file extension or content-type header.");
    }

    private static PartTypeInfo MimeToContentType(string mime)
    {
        if (TryMimeToContentType(mime, out var ct)) return ct;
        throw new ArgumentException($"Unsupported MIME type: {mime}. Supported: image/png, image/jpeg, image/gif, image/bmp, image/tiff, image/svg+xml");
    }

    private static bool TryMimeToContentType(string mime, out PartTypeInfo contentType)
    {
        contentType = mime.ToLowerInvariant() switch
        {
            "image/png" => ImagePartType.Png,
            "image/jpeg" or "image/jpg" => ImagePartType.Jpeg,
            "image/gif" => ImagePartType.Gif,
            "image/bmp" => ImagePartType.Bmp,
            "image/tiff" or "image/tif" => ImagePartType.Tiff,
            "image/svg+xml" => ImagePartType.Svg,
            "image/emf" or "image/x-emf" => ImagePartType.Emf,
            "image/wmf" or "image/x-wmf" => ImagePartType.Wmf,
            _ => default
        };
        return contentType != default;
    }

View on GitHub (pinned to 1ced45e900)

Solutions

Convert the image to a supported format (png, jpeg, gif, bmp, tiff, svg) before creating the data URI.
If the MIME type is a variant of a supported format, use the canonical form (e.g. 'image/jpeg' not 'image/jpegn').
For EMF/WMF, use the MIME types 'image/emf'/'image/x-emf' or 'image/wmf'/'image/x-wmf'.

Example fix

// before — WebP MIME not supported
add image src='data:image/webp;base64,UklGRkR...' path='/body'

// after — convert to PNG, use PNG MIME
add image src='data:image/png;base64,iVBORw0KGgo...' path='/body'

Defensive patterns

Strategy: validation

Validate before calling

// Pre-check the MIME type is supported before resolving
var supportedMimes = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
{
    "image/png", "image/jpeg", "image/jpg", "image/gif",
    "image/bmp", "image/tiff", "image/tif", "image/svg+xml",
    "image/emf", "image/x-emf", "image/wmf", "image/x-wmf"
};

string mime = ExtractMimeFromDataUri(source); // your helper
if (!supportedMimes.Contains(mime))
{
    Console.Error.WriteLine($"Unsupported MIME type: {mime}. Convert to a supported format.");
    return;
}

Try / catch

try
{
    var (stream, contentType) = ImageSource.Resolve(source);
}
catch (ArgumentException ex) when (ex.Message.Contains("Unsupported MIME type"))
{
    // Convert the image to png/jpeg and rebuild the data URI
}

Prevention

Convert images to standard formats (png, jpeg) before creating data URIs.
Validate the MIME type in the data URI header against the supported set before embedding.
Avoid uncommon or vendor-specific MIME types for embedded images.

When it happens

Trigger: Passing a data URI with an unsupported MIME type like 'data:image/webp;base64,...'. When ResolveDataUri extracts the MIME from the header and calls MimeToContentType which throws for unrecognized types. When ResolveUrl reads a server Content-Type of 'image/heic' and TryMimeToContentType returns false, but the caller uses the throwing variant.

Common situations: A data URI generated from a WebP, HEIC, AVIF, or ICO image — formats not in the supported set. A server that reports an uncommon or vendor-specific MIME type for a standard image. A user who hand-crafted a data URI with a typo in the MIME type (e.g. 'image/jpegn').

Related errors

AI-assisted analysis of iOfficeAI/OfficeCLI@1ced45e900 (2026-08-13). Data as JSON: /api/errors/2db6b7f364f918fd. Report an issue: GitHub.