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Why are some raster files not downloadable from image services on ArcGIS Server .NET edition?
By default, IIS only serves known MIME types. Many raster file extensions are not in this list, for example, .jp2, .img, .sid, etc. Downloading these files fails with the HTTP Error 404.3 error.
To make them downloadable, these extensions must be added to MIME type list of the virtual arcgisoutput directory (or virtual download directory if used). To edit MIME types, refer to Microsoft knowledge base article 326965: IIS 6.0 does not serve unknown MIME types for details.