API Proposal: Media components (Image, Video, FileDownload)

## Background and Motivation

Blazor has no built-in way to display images, videos, or trigger file downloads from non-HTTP sources (byte arrays, streams, database BLOBs). Developers currently must base64-encode data (causing large string allocations) or create separate HTTP endpoints (which can interfere with authorization). Issue #25274 (20+ upvotes) has requested an Image component since 2020. This feature introduces a suite of media components using streaming JS interop and browser Cache Storage API for efficient data transfer.

PRs #63360 (Image), #63540 (Media Suite — Video, FileDownload, MediaComponentBase, MediaSource, MediaContext), and #67130 (extract media components into a standalone `Microsoft.AspNetCore.Components.Media` package). Related to issue #25274.

## Proposed API

The media components ship in a new standalone package, `Microsoft.AspNetCore.Components.Media`, under the matching `Microsoft.AspNetCore.Components.Media` namespace. The package depends only on `Microsoft.AspNetCore.Components` and `Microsoft.JSInterop` — it has no dependency on `Microsoft.AspNetCore.Components.Web`, so it can be consumed from any Blazor renderer (Web, native, custom hosts).

```diff
+ namespace Microsoft.AspNetCore.Components.Media;
+
+ public class MediaSource
+ {
+ public MediaSource(byte[] data, string mimeType, string cacheKey);
+ public MediaSource(Stream stream, string mimeType, string cacheKey);
+ public string CacheKey { get; }
+ public long? Length { get; }
+ public string MimeType { get; }
+ public Stream Stream { get; }
+ }
+
+ public class MediaContext
+ {
+ public ElementReference Element { get; set; }
+ public bool HasError { get; }
+ public bool IsLoading { get; }
+ public string? ObjectUrl { get; }
+ }
+
+ public abstract class MediaComponentBase : ComponentBase, IAsyncDisposable
+ {
+ public Dictionary? AdditionalAttributes { get; set; }
+ public MediaSource Source { get; set; }
+ public ValueTask DisposeAsync();
+ }
+
+ public class Image : MediaComponentBase
+ {
+ public RenderFragment? ChildContent { get; set; }
+ }
+
+ public class Video : MediaComponentBase
+ {
+ public RenderFragment? ChildContent { get; set; }
+ }
+
+ public class FileDownload : ComponentBase
+ {
+ public RenderFragment? ChildContent { get; set; }
+ public string FileName { get; set; }
+ public string? Text { get; set; }
+ }
+
+ public class FileDownloadContext
+ {
+ public string FileName { get; }
+ public Task InvokeAsync();
+ }
```

## Packaging

- **New package:** `Microsoft.AspNetCore.Components.Media`
- **Namespace:** `Microsoft.AspNetCore.Components.Media` (changed from the originally proposed `Microsoft.AspNetCore.Components.Web.Media`)
- **Dependencies:** `Microsoft.AspNetCore.Components`, `Microsoft.JSInterop` (no dependency on `Microsoft.AspNetCore.Components.Web`)
- Consumers add a `PackageReference` to `Microsoft.AspNetCore.Components.Media` and an `@using Microsoft.AspNetCore.Components.Media` to pick up the components.

## Usage Examples

Image from byte array:

```razor
@using Microsoft.AspNetCore.Components.Media

```

Video from stream:

```razor
@using Microsoft.AspNetCore.Components.Media

```

File download:

```razor
@using Microsoft.AspNetCore.Components.Media

```

## Alternative Designs

Base64 encoding (allocates large strings), creating separate HTTP endpoints (interferes with authorization, requires more work). The streaming approach avoids large .NET-side memory allocations and uses browser Cache Storage for repeat loads.

## Risks

MediaSource is single-use (one instance per media load). The browser Cache Storage API is used for caching, which has storage limits depending on the browser.