Simple Sitecore/ASP.NET page to check what IFilters are known to the Sitecore system. It will also attempt to locate a media library item of each type and read text out of that file. This should help troubleshoot why some content inside of media library items are not being indexed.

Sitecore-IFilters.aspx

<%@ Page Language="C#" %>

<script runat="server">
    protected void Page_Load(object sender, EventArgs e)
    {
        // These are the extensions that Sitecore 7 supports indexing content for
        // See: Sitecore.ContentSearch.ComputedFields.MediaItemContentExtractor
        string[] extensions = new[]{ ".pdf",
                            ".html", 
                            ".rtf",
                            ".odt",
                            ".doc",
                            ".dot",
                            ".docx",
                            ".dotx",
                            ".docm",
                            ".dotm",
                            ".xls",
                            ".xlt",
                            ".xla",
                            ".xlsx",
                            ".xltx",
                            ".xlsm",
                            ".xltm",
                            ".xlam",
                            ".xlsb",
                            ".ppt",
                            ".pot",
                            ".pps",
                            ".ppa",
                            ".pptx",
                            ".potx",
                            ".ppsx",
                            ".ppam",
                            ".pptm",
                            ".potm",
                            ".ppsm"};

        System.Collections.Generic.List<IFilterRegistration> registrations = GetRegistrations(extensions).ToList();

        InstalledFilters.DataSource = registrations;
        InstalledFilters.DataBind();

         
        //Try the FilterReader class read a few characters
        char[] chars = new char[10];
        using (Sitecore.ContentSearch.Extracters.IFilterTextExtraction.FilterReader reader = new Sitecore.ContentSearch.Extracters.IFilterTextExtraction.FilterReader(Server.MapPath("~/temp/Word-2010.docx")))
        {
            reader.Read(chars, 0, 5);
        }

        Response.Write(string.Join("", chars));
        
    }

    protected System.Collections.Generic.IEnumerable<IFilterRegistration> GetRegistrations(params string[] extentions)
    {
        System.Type filterLoader = System.Type.GetType("Sitecore.ContentSearch.Extracters.IFilterTextExtraction.FilterLoader, Sitecore.ContentSearch");

        //  private static bool GetFilterDllAndClass(string ext, out string dllName, out string filterPersistClass)
        System.Reflection.MethodInfo dynMethod = filterLoader.GetMethod("GetFilterDllAndClass", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);

        foreach (string extension in extentions)
        {
            string dllName = null;
            string filterPersistClass = null;
            object[] args = new object[] { extension, dllName, filterPersistClass };
            dynMethod.Invoke(null, args);

            System.Reflection.MethodInfo loadFilterFromDllMethod = filterLoader.GetMethod("LoadFilterFromDll", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);
            Sitecore.ContentSearch.Extracters.IFilterTextExtraction.IFilter ifilter = loadFilterFromDllMethod.Invoke(null, new object[] { dllName, filterPersistClass }) as Sitecore.ContentSearch.Extracters.IFilterTextExtraction.IFilter;

            if (ifilter == null)
            {
                throw new Exception("ifilter == null");
            }
            
            var extractor = new Sitecore.ContentSearch.ComputedFields.MediaItemIFilterTextExtractor();
            if (extractor == null)
            {
                throw new Exception("Extractor is null!");
            }

            Sitecore.Data.Items.Item item = GetMediaItem(extension);
            string content = "";
            string sampleItemPath = "Item not found.";
            if (item != null)
            {
                sampleItemPath = item.Paths.Path;
                var indexableItem = (Sitecore.ContentSearch.SitecoreIndexableItem)item;
                try
                {
                    object value = extractor.ComputeFieldValue(indexableItem);
                    content = (value ?? "").ToString();
                }
                catch(Exception e)
                {
                    content = string.Concat("Exception trying to execute: ", e.Message);
                }
            }

            yield return new IFilterRegistration
            {
                Extension = extension,
                DLL = (args[1] ?? "").ToString(),
                FilterPersistClass = (args[2] ?? "").ToString(),
                SampleItem = sampleItemPath,
                SampleContent = content
            };
        }
    }

    // Looks in Sitecore for a media library item of a certain type
    protected Sitecore.Data.Items.Item GetMediaItem(string extension)
    {
        extension = extension.TrimStart('.');
        using (var context = Sitecore.ContentSearch.ContentSearchManager.GetIndex("sitecore_master_index").CreateSearchContext())
        {
            Sitecore.ContentSearch.SearchTypes.SearchResultItem sri = context.GetQueryable<Sitecore.ContentSearch.SearchTypes.SearchResultItem>()
                //.Where(i => i.Path.StartsWith("/sitecore/media library", StringComparison.OrdinalIgnoreCase))
                .Where(i => string.Equals(i["Extension"], extension, StringComparison.OrdinalIgnoreCase))
                .FirstOrDefault();

            if (sri != null)
            {
                return sri.GetItem();
            }
        }
        return null;
    }

    protected class IFilterRegistration
    {
        public string Extension { get; set; }

        public string DLL { get; set; }

        public string FilterPersistClass { get; set; }

        public string SampleItem { get; set; }
        
        public string SampleContent { get; set; }
    }

</script>
<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            ContentSearchConfigurationSettings.MediaIndexingFolder = <%=Sitecore.ContentSearch.Utilities.ContentSearchConfigurationSettings.MediaIndexingFolder %>
            <h1>Known IFilters</h1>
            <table border="1">
                <asp:Repeater ID="InstalledFilters" runat="server">
                    <ItemTemplate>
                        <tr>
                            <td><%#Eval("Extension") %></td>
                            <td><%#Eval("DLL") %></td>
                            <td><%#Eval("FilterPersistClass") %></td>
                            <td><%#Eval("SampleItem") %></td>
                            <td><%#Eval("SampleContent") %></td>
                        </tr>
                    </ItemTemplate>
                </asp:Repeater>
            </table>
        </div>
    </form>
</body>
</html>

How are you instantiating Sitecore.ContentSearch.Extracters.IFilterTextExtraction.IFilter? This is a protected class in 7.5, also in 7.1. Thanks!

You need to break up the gist a little: make an aspx page that inherits a class with the contents of the <script> tag as it's body. Put the class in a project that compiles with the assembly name "Sitecore.ContentSearch.UnitTests". This way, the internal types will be visible to your project.