cefsharp/CefSharp · error · ArgumentNullException

Please provide a valid mimeType

Error message

Please provide a valid mimeType

What it means

Thrown by the FileResourceHandler constructor when mimeType is null or empty. FileResourceHandler serves a file from disk to CEF via CefStreamReader::CreateForFile; CEF needs a valid MIME type to set response headers, so the constructor rejects an undefined type with ArgumentNullException.

Source

Thrown at CefSharp/Internals/FileResourceHandler.cs:39

        /// Path of the underlying file
        /// </summary>
        public string FilePath { get; private set; }

        /// <summary>
        /// Gets or sets the Mime Type.
        /// </summary>
        public string MimeType { get; set; }

        /// <summary>
        /// Initializes a new instance of the <see cref="FileResourceHandler"/> class.
        /// </summary>
        /// <param name="mimeType">mimeType</param>
        /// <param name="filePath">filePath</param>
        public FileResourceHandler(string mimeType, string filePath)
        {
            if (string.IsNullOrEmpty(mimeType))
            {
                throw new ArgumentNullException("mimeType", "Please provide a valid mimeType");
            }

            if (string.IsNullOrEmpty(filePath))
            {
                throw new ArgumentNullException("filePath", "Please provide a valid filePath");
            }

            if (!File.Exists(filePath))
            {
                throw new FileNotFoundException("Unable to create FileResourceHandler", filePath);
            }

            MimeType = mimeType;
            FilePath = filePath;
        }

        bool IResourceHandler.ProcessRequest(IRequest request, ICallback callback)
        {

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Pass an explicit non-empty MIME type such as "text/html" or "application/pdf".
2. Resolve the type from the file extension via a MIME map with a default fallback.
3. Guard: if (string.IsNullOrEmpty(mimeType)) mimeType = "application/octet-stream"; before constructing.

Example fix

// before
return new FileResourceHandler(mimeType, filePath);

// after
if (string.IsNullOrEmpty(mimeType))
    mimeType = MimeMapping.GetMimeMap(Path.GetExtension(filePath)) ?? "application/octet-stream";
return new FileResourceHandler(mimeType, filePath);

if (string.IsNullOrEmpty(mimeType))
{
    mimeType = "application/octet-stream";
}
return new FileResourceHandler(mimeType, filePath);

When it happens

Trigger: Constructing new FileResourceHandler(null, path) or new FileResourceHandler("", path) in a GetResourceHandler implementation that serves local files. The MIME type is often derived from the extension and may be unset.

Common situations: Serving local HTML/JSON/PDF assets where the MIME type is computed at runtime and returned null; copy-paste that omitted the argument; serving a file whose extension has no mapped type.

Related errors

• Please provide a valid mimeType
• Please provide a valid array
• Unable to load byte[] with length 0.
• Please provide a valid filePath
• manager

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/32dd329215955144. Report an issue: GitHub.