Add explainer for "write" permission mode

proposals/WriteMode.md

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+# A "write"-only permission mode for the File System Access API
+
+This explainer describes a proposed "write" permission mode for the [File System Access API](https://wicg.github.io/file-system-access/).
+
+## Authors
+
+* [Ming-Ying Chung](https://github.com/mingyc) - Google
+
+## Introduction
+
+The File System Access API allows web applications to read, write, and manage files on the local file system, with the user's permission. It provides a powerful way for web apps to interact with files in a way that was previously only possible for native applications.
+
+Currently, the API defines two access modes for file system entries: `"read"` and `"readwrite"`. These modes are used when an application queries for or requests permission to a file or directory.
+
+## Motivation
+
+The problem with the current permission model is that operations requiring only file system modification, such as `FileSystemHandle.remove()`, are forced to request the broad `"readwrite"` permission. This is not ideal from a security perspective, as it grants more permission than necessary.
+
+To address this, we propose a more granular, "write-only" permission mode.
+
+## Proposed Solution
+
+We propose adding a new `"write"` permission mode to the `FileSystemPermissionMode` enum in the File System Access API.
+
+```javascript
+enum FileSystemPermissionMode {
+  "read",
+  "readwrite",
+  "write" // new
+};
+```
+
+This new mode will allow applications to request only the permission to write to a file or directory, without also gaining permission to read from it. This follows the principle of least privilege, enhancing security.
+
+Several existing methods that currently request `"readwrite"` permission will be updated to use the more appropriate permission level. Operations that only modify or create files will now be able to request `"write"` permission instead.
+
+Key changes include:
+
+*   **`FileSystemHandle.remove()`**: This operation will now only require `"write"` permission.
+*   **`FileSystemFileHandle.createWritable()`**: Creating a writable stream to a file will now be a `"write"`-only operation.
+*   **`FileSystemDirectoryHandle.getFileHandle({create: true})`**: Creating a new file handle will require `"write"` permission.
+*   **`FileSystemDirectoryHandle.getDirectoryHandle({create: true})`**: Creating a new directory handle will require `"write"` permission.
+*   **`FileSystemDirectoryHandle.removeEntry()`**: This operation will now only require `"write"` permission.
+
+## Developer Experience
+
+The introduction of the `"write"` mode is designed to be largely transparent to developers. It provides a clearer and more secure permission model without requiring changes to existing code.
+
+While developers will not need to change their code to adopt this, the permission prompts shown to the user may become more specific, and the browser will enforce a stricter, more secure permission grant. Sites with existing already granted permissions should continue to work as before.
+
+For example, when deleting a file:
+
+```javascript
+// Assume 'dirHandle' is a FileSystemDirectoryHandle the user has granted access to.
+
+// In the old model, this would require "readwrite" permission.
+// With this change, it will only require "write" permission on the directory.
+await dirHandle.removeEntry('file-to-delete.txt');
+```
+
+Similarly, when creating a file and writing to it:
+
+```javascript
+// Assume 'dirHandle' is a FileSystemDirectoryHandle.
+// This operation will now only require "write" permission.
+const newFileHandle = await dirHandle.getFileHandle('new-file.txt', { create: true });
+
+// Creating a writer will also fall under the "write" permission grant.
+const writable = await newFileHandle.createWritable();
+await writable.write('Some content');
+await writable.close();
+```
+
+## Security and Privacy Considerations
+
+The primary motivation for this change is to improve security. By introducing a "write"-only mode, we can ensure that web applications are granted the minimum level of privilege necessary to perform an operation. This aligns the API with the principle of least privilege.
+
+## Stakeholder Feedback / Opposition
+
+*   Gecko : No signals.
+*   WebKit : No signals.
+*   Web developers: No signals.
+*   Google: This is a Google proposal.