Documentation: Document which file formats exist

I intentionally don't document the format specifics, to prevent this
file from becoming outdated. Hence, only a list with pointers to where
to find details.

Documentation/FileFormats.md

@@ -0,0 +1,100 @@
+# Serenity Formats
+
+This document summarizes all non-standard file formats, data formats, and mime-types used by Serenity.
+
+## Clipboard
+
+The clipboard feature works through the Clipboard server, which generally acts as a global storage or three things:
+- a `String` mime type,
+- a (potentially large) block of data, shared as an anonymous file,
+- a `HashMap<String, String>` of arbitrary metadata, depending on the mime type.
+
+See also [`Userland/Libraries/LibGUI/Clipboard.h`](../Userland/Libraries/LibGUI/Clipboard.h).
+
+## Drag & drop
+
+In contrast to the clipboard, the drag & drop feature works through WindowServer, and a bouquet of data is transmitted:
+- a `[UTF8] String` to be displayed while dragging,
+- a `HashMap<String, ByteBuffer>` map that contains arbitrary data for a variety of possible mime types,
+- a `Gfx::ShareableBitmap` to be displayed while dragging
+
+Drag & drop is most prominently supported by File Manager, Spreadsheet, and Terminal.
+Various applications accept drag & drop to open files.
+
+## glyph/x-fonteditor (Clipboard-only)
+
+Requires the metadata-fields, `char` (usually encoded as a string of length 1: the codepoint as a character),
+`width`, and `height` (encoded as a decimal number).
+
+The glyph bitmap data is encoded in width times height many bytes, either 0 (clear) or 1 (set).
+
+Implemented in `GlyphEditorWidget::copy_glyph` and `GlyphEditorWidget::paste_glyph`, in [`Userland/Applications/FontEditor/GlyphEditorWidget.cpp`](../Userland/Applications/FontEditor/GlyphEditorWidget.cpp).
+
+## image/x-serenityos (Clipboard-only)
+
+Requires the metadata-fields `width`, `height`, `scale`, `format` (see `Gfx::BitmapFormat`), and `pitch`, encoded as decimal strings.
+
+The data is encoded according to `Gfx::determine_storage_format(BitmapFormat)`, so either as
+BGRx8888, BGRA8888, RGBA8888, or 8-bit palette index. Note that the palette is not transferred.
+
+Implemented in [`Clipboard::set_bitmap` and `Clipboard::DataAndType::as_bitmap()`](../Userland/Libraries/LibGUI/Clipboard.cpp).
+
+## text/uri-list (Clipboard and drag&drop)
+
+Newline-delimited set of URIs. Used by File Manager, FileSystemModel, and Terminal.
+
+Example:
+
+```
+file:///home/anon/Desktop/Browser
+file:///home/anon/Desktop/Help
+file:///home/anon/Desktop/Home
+```
+
+## Application File (`*.af` files)
+
+These files are human-readable and are a subset of the INI-format, have no easily detectable filemagic.
+These files define System Menu entries and launcher file types / protocols.
+
+They are stored in [`/res/apps`](../Base/res/apps).
+
+See also [`Userland/Services/Taskbar/main.cpp`](../Userland/Services/Taskbar/main.cpp) and `Launcher::load_handlers` in [`Userland/Services/LaunchServer/Launcher.cpp`](../Userland/Services/LaunchServer/Launcher.cpp).
+
+## Font (`*.font` files)
+
+These files contain bitmap definitions of fonts, either varying-width or fixed-width.
+The header definition can be found in `Gfx::FontFileHeader` in [`Userland/Libraries/LibGfx/BitmapFont.cpp`](../Userland/Libraries/LibGfx/BitmapFont.cpp).
+Most prominently, the first four bytes contain the filemagic: `!Fnt`.
+
+## GUI Markup Language (`*.gml` files)
+
+These files are human-readable, have no easily detectable filemagic, and define GUI interfaces.
+The format is strongly influenced by QML, the Qt Modeling Language.
+
+See also [Playground(1)](../Userland/DevTools/Playground/) and `Widget::load_from_gml` in [`Userland/Libraries/LibGUI/Widget.cpp`](../Userland/Libraries/LibGUI/Widget.cpp).
+
+## Inter Process Communication (`*.ipc` files)
+
+These files are human-readable, have no easily detectable filemagic, and define IPC interfaces.
+The format is loosely inspired by C++ headers.
+
+See also [`Meta/Lagom/Tools/CodeGenerators/IPCCompiler/`](../Meta/Lagom/Tools/CodeGenerators/IPCCompiler/).
+
+## Inter Process Communication (through Unix sockets)
+
+The various services each have their own formats, all very similar, and automatically implemented through LibIPC. The specifics depend on the corresponding source `.ipc` file.
+
+The format can be identified by the format magic, which is derived in [`Meta/Lagom/Tools/CodeGenerators/IPCCompiler/main.cpp`](../Meta/Lagom/Tools/CodeGenerators/IPCCompiler/main.cpp)
+from the service-endpoint name, e.g. "ClipboardClient" (which hashes to 4008793515) or "ClipboardServer" (which hashes to 1329211611).
+
+In general, communication works by packets, which might have been sent in response to other packets. Everything is host endianness. Each packet consists of:
+- a 32-bit message size (see `Connection::try_parse_messages` in [`Userland/Libraries/LibIPC/Connection.h`](../Userland/Libraries/LibIPC/Connection.h))
+- the 32-bit endpoint magic (note that responses use the endpoint of the requesting packet, so the Clipboard server might use the endpoint magic 4008793515 to signal that this packet is a response)
+- the 32-bit message ID within that endpoint (sequentially assigned, starting at 1)
+- the data of that message itself (e.g. see `Messages::ClipboardServer::SetClipboardData::{en,de}code` in `Build/*/Userland/Services/Clipboard/ClipboardServerEndpoint.h`).
+
+## Postcreate (`*.postcreate` files)
+
+Shell-script executed by HackStudio after creating a new project.
+
+See also `ProjectTemplate::create_project` in [`Userland/DevTools/HackStudio/ProjectTemplate.cpp`](../Userland/DevTools/HackStudio/ProjectTemplate.cpp).