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I have accomplished this before using HttpHandlers by adding tags to web.xml accordingly. I might as well copy the code here..

private readonly string[] SupportedExtensions = new[] { "*.docx", ".pptx", "xlsx" };
private void AddForSharePoint2007()
    {
        // SP2007 uses IIS6.0 compatibility mode
        SPWebService contentService = SPWebService.ContentService;

        // the Name must be XPATH of Value.. otherwise SP wont remove!
        SPWebConfigModification item = new SPWebConfigModification
        {
            Owner = FeatureId,
            Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode,
            Path = "/configuration/system.web/httpHandlers",
            Name = "add[@verb=\"*\"][@path=\"" + string.Join(",", SupportedExtensions) + "\"][@type=\"MyNamespace.DownloadHandler, MyProject, Version=1.0.0.0, Culture=neutral, PublicKeyToken=*****\"]",
            Value = "<add verb=\"*\" path=\"" + string.Join(",", SupportedExtensions) + "\" type=\"MyNamespace.DownloadHandler, MyProject, Version=1.0.0.0, Culture=neutral, PublicKeyToken=******\"/>"
        };

        contentService.WebConfigModifications.Add(item);
        contentService.Update();
        contentService.ApplyWebConfigModifications();
    }


    private void AddForSharePoint2010()
    {
        // SP2010 requires IIS7.0
        const string valueTemplate = "<add name=\"DownloadHandler for {0}\" path=\"{1}\" verb=\"*\" type=\"MyNamespace.DownloadHandler, MyProject, Version=4.13.0.0, Culture=neutral, PublicKeyToken=******\"/>";
        const string xpathTemplate = "add[@name=\"DownloadHandler for {0}\"][@path=\"{1}\"][@verb=\"*\"][@type=\"MyNamespace.DownloadHandler, MyProject, Version=4.13.0.0, Culture=neutral, PublicKeyToken=******\"]";
        SPWebService contentService = SPWebService.ContentService;

        foreach (var extension in SupportedExtensions)
        {
            // the Name must be XPATH of Value.. otherwise SP wont remove!
            SPWebConfigModification item = new SPWebConfigModification()
            {
                Owner = FeatureId,
                Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode,
                Path = "/configuration/system.webServer/handlers",
                Name = string.Format(xpathTemplate,extension.Replace("*.", string.Empty), extension),
                Value = string.Format(valueTemplate, extension.Replace("*.", string.Empty), extension)
            };

            contentService.WebConfigModifications.Add(item);
        }

        contentService.Update();
        contentService.ApplyWebConfigModifications();
    }

As it can be seen from SupportedExtensions list, the HTTPHandler is called for those file extensions. Yet this time, I need it to intercept all file downloads from document libraries in Sharepoint. I could simply define SupportedExtensions as "*", but then the handler will be called for each HTTP request on Sharepoint including css, js, image, html request; which will lead to huge performance problems. That is obviously not acceptable.

So my question is: Is it possible to register and implement an HTTPHandler for only file requests from Document Libraries? Or writing a very efficient handler where it can distinguish a file request from Sharepoint's own files?

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