"Extension Methods" lists generic methods whose type arguments or constraints do not match

The `System.Net.Http.MultipartContent` class implements `IEnumerable`. The documentation of this class lists many extension methods that are applicable to some `IEnumerable` but not to `IEnumerable`. Those extension methods should not be listed here because they cannot be used on `MultipartContent`. For example:

- `CopyToDataTable (this IEnumerable source) where T : DataRow`, because `HttpContent` is not derived from `DataRow`
- `Average (this IEnumerable source)`, because `MultipartContent` does not implement `IEnumerable`
- `Ancestors (this IEnumerable source) where T : XNode`, because `HttpContent` is not derived from `XNode`

Listing those inapplicable extension methods makes it harder to find the methods that actually can be used.

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#### Document Details

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* ID: 1c23630f-1e62-f2ed-3697-ed01b305d96b
* Version Independent ID: ac5fbfde-d2b1-945d-fd7b-42fb30566209
* Content: [MultipartContent Class (System.Net.Http)](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.multipartcontent?view=netframework-4.7.2#extension-methods)
* Content Source: [xml/System.Net.Http/MultipartContent.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Http/MultipartContent.xml)
* Product: **dotnet-api**
* GitHub Login: @karelz
* Microsoft Alias: **ncldev**