# Base64 → File

A single-file web tool that decodes a base64 string and saves the result back out as a real file. Useful for pulling attachments out of API responses, log dumps, JSON payloads, config blobs and `data:` URIs without hunting for a working online decoder — and without pasting anything sensitive into someone else's website.

## What it does

Open `base64-decoder.html` in any browser, paste the base64, and the decoded file appears on the right — type detected, previewed, ready to download.

Everything runs locally in the page. There is no server, no build step, no dependencies, and nothing is ever uploaded.

```bash
xdg-open base64-decoder/base64-decoder.html
```

## Input handling

The input is cleaned up before decoding, so most real-world paste jobs work as-is:

| Input | Handled |
|---|---|
| `data:image/png;base64,iVBORw0...` | Prefix stripped, MIME type read from the header |
| Line-wrapped / indented base64 | All whitespace removed |
| URL-safe base64 (`-` and `_`) | Converted to standard `+` and `/` |
| Missing `=` padding | Restored |
| Percent-encoded (`%2B`) | URL-decoded first |
| Wrapping quotes from a JSON value | Trimmed |

Invalid input gives a specific reason — the offending character, or a truncated length — rather than a generic failure.

## Type detection

The file type is worked out in three passes, in order of reliability:

1. **Magic bytes** — ~30 signatures covering PNG, JPEG, GIF, WEBP, BMP, ICO, PDF, ZIP, GZIP, BZ2, XZ, 7z, RAR, TAR, MP3, MP4, MKV, OGG, FLAC, WAV, AVI, SQLite, TTF/OTF/WOFF/WOFF2, ELF, EXE and Java class files
2. **Data URI header** — if the input was a `data:` URI, its declared MIME type supplies the extension
3. **Content sniffing** — payloads that decode as valid UTF-8 are checked for SVG, HTML, XML, JSON, PEM and shell scripts

Anything unrecognised falls back to `application/octet-stream` and a `.bin` extension. The suggested filename is always editable before saving.

## Preview

| Decoded type | Preview |
|---|---|
| Images | Rendered inline |
| Audio / video | Playable with controls |
| PDF | Embedded viewer |
| Text-based | Syntax-free text view, plus a **Copy text** button |
| Anything else | Hex dump of the first 192 bytes with an ASCII column |

## Shortcuts

- **Drag and drop** a `.txt` / `.b64` file onto the input to load its contents
- **Ctrl/Cmd + S** downloads the decoded file
- **Enter** in the filename field downloads immediately

## Requirements

Any modern browser. Works straight from `file://` — no web server needed.

The **Paste** button uses the clipboard API, which some browsers restrict on `file://` pages; if it does nothing, use Ctrl+V in the input instead.