Hi Chris,

Thank you for the feature request! We have implemented support for the following image file extensions in RadEditor's ImageManager and RadFileExplorer starting with the 2026 Q2 release:

• .avif
• .heif
• .heic
• .webp

What's included

• These extensions are now part of the default SearchPatterns — files in these formats will automatically appear in the Image Manager file browser without any additional RadEditor configuration.
• AVIF and WebP images render full thumbnail previews in the Image Manager's Thumbnail View, as they are natively supported by all modern browsers.
• HEIF and HEIC files are listed with a proper image file icon in Grid View. Thumbnail previews for these formats are not available because Chrome, Edge, and Firefox do not support HEIF/HEIC rendering natively — only Safari does. If cross-browser preview is important, we recommend converting HEIF/HEIC files to AVIF or WebP on the server side.

Note: We could not find a recognized image format called "abci". If you have more details about this format, please let us know and we'll evaluate adding it.

Required: IIS MIME Type Configuration

IIS does not serve .avif, .heif, .heic, and .webp files by default. Without registering the MIME types, the browser will receive 404 (Not Found) errors when loading these images. Add the following to your application's web.config:

<configuration>
  <system.webServer>
    <staticContent>
      <remove fileExtension=".avif" />
      <mimeMap fileExtension=".avif" mimeType="image/avif" />
      <remove fileExtension=".heif" />
      <mimeMap fileExtension=".heif" mimeType="image/heif" />
      <remove fileExtension=".heic" />
      <mimeMap fileExtension=".heic" mimeType="image/heic" />
      <remove fileExtension=".webp" />
      <mimeMap fileExtension=".webp" mimeType="image/webp" />
    </staticContent>
  </system.webServer>
</configuration>

The <remove> entries prevent "duplicate collection" errors in case the extension is already registered at the machine level.

This feature will be available in the Telerik UI for ASP.NET AJAX 2026 Q2 release.

Regards,
Rumen
Progress Telerik