# detection/from-extension

## Purpose

Detects the MIME type from a file extension or full filename. Strips path components and leading dots, normalizes to lowercase, and looks up the result in the built-in MIME database. Returns a `DetectionResult` with `source: "extension"` and a confidence weight of 0.7 in the unified `detect()` function.

## Features

- Accepts both raw extensions (`.png`, `png`) and full filenames (`photo.jpeg`)
- Strips directory path components automatically
- Normalizes extensions to lowercase before lookup
- Handles multiple dots in filenames correctly (uses the last extension)
- Returns `undefined` for unknown extensions instead of throwing
- Wraps the database entry in a `DetectionResult` envelope

## Usage: Basic Example

```typescript
import { detectFromExtension } from "@suaveplan/mime/detection/from-extension";

const result = detectFromExtension("photo.jpg");
console.log(result?.mime);   // "image/jpeg"
console.log(result?.source); // "extension"

// Just an extension without filename
const svg = detectFromExtension(".svg");
console.log(svg?.mime); // "image/svg+xml"
```

## Usage: Advanced Example

```typescript
import { detectFromExtension } from "@suaveplan/mime/detection/from-extension";

function getMimeForUpload(filename: string): string {
    const detected = detectFromExtension(filename);
    // Fall back to binary stream for unknown extensions
    return detected?.mime ?? "application/octet-stream";
}

// Handles paths with directories
console.log(detectFromExtension("/uploads/2024/report.pdf")?.mime); // "application/pdf"

// Unknown extension returns undefined
console.log(detectFromExtension("file.xyz")); // undefined
```

## API Reference

### `detectFromExtension(input: string): DetectionResult | undefined`

| Parameter | Type | Description |
|-----------|------|-------------|
| `input` | `string` | File extension (with or without dot) or full filename/path. |

**Returns:** `DetectionResult` with `source: "extension"` and the matched `mime`, or `undefined` when the extension is not in the database.

**Throws:** Never.

## Implementation Notes

The function extracts the extension by splitting on `.` and taking the last segment, then looking up `getEntryByExtension`. The split also handles filenames with no extension (returns `undefined`) and filenames starting with a dot (treated as hidden files with no extension, e.g. `.gitignore`). Path components are stripped by looking only at the basename portion after the last `/` or `\`. Lookup is O(1) via the registry's `Map<string, MimeEntry>` index.