arc: Use the MIME type returned by the container to handle content URLs

components/arc/common/file_system.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
-// Next MinVersion: 4
+// Next MinVersion: 5
 
 module arc.mojom;
 
 // Represents a document in Android DocumentsProvider.
 // See Android docs of DocumentsContract.Document for details.
 struct Document {
   // Opaque ID of the document.
   string document_id;
 
   // Display name of the document.
@@ skipping 28 matching lines @@
 };
 
 
 // Next method ID: 1
 interface FileSystemHost {
   // Called when a watched document was changed.
   // |type| describes the type of change made to the document.
   [MinVersion=3] OnDocumentChanged@0(int64 watcher_id, ChangeType type);
 };
 
-// Next method ID: 8
+// Next method ID: 9
 interface FileSystemInstance {
   // Notes about Android Documents Provider:
   //
   // In Android Storage Access Framework, a document is uniquely identified by
   // a pair of "authority" and "document ID".
   //
   // - An authority specifies a Documents Provider that serves a document.
   //   It is the origin part of a content:// URI used to access the Documents
   //   Provider via Content Resolver protocol.
   //   Example: "com.android.providers.media.documents"
@@ skipping 33 matching lines @@
   // Documents Provider.
   // If such a document does not exist, null is returned.
   [MinVersion=2] GetDocument@3(string authority, string document_id) =>
       (Document? document);
 
   // Asks the ContentResolver for the size of the file specified by the URL.
   // If the file does not exist or the size is unknown (e.g. directories and
   // streams), -1 is returned.
   [MinVersion=1] GetFileSize@1(string url) => (int64 size);
 
+  // Asks the ContentResolver to get the MIME type of the file specified by the
+  // URL.

[dcheng, 2017/06/01 11:06:44]

Nit: document what it means if nothing is returned (and if that's distinct from
the empty string)

[hashimoto, 2017/06/01 11:30:16]

Done.

+  [MinVersion=4] GetMimeType@8(string url) => (string? mime_type);
+
   // Establishes full-duplex communication with the host.
   [MinVersion=3] Init@5(FileSystemHost host_ptr);
 
   // Asks the ContentResolver to get a FD to read the file specified by the
   // URL.
   [MinVersion=1] OpenFileToRead@2(string url) => (handle? fd);
 
   // Uninstalls a document watcher.
   //
   // After this method call returns, OnDocumentChanged() will never be called
   // with the watcher ID. Whether OnDocumentChanged() is called or not after
   // this method is called and before this method returns is undefined.
   //
   // It fails if the specified watcher does not exist.
   [MinVersion=3] RemoveWatcher@7(int64 watcher_id) => (bool success);
 
   // Requests MediaProvider to scan specified files.
   // When the specified file does not exist, the corresponding entry in
   // MediaProvider is removed.
   RequestMediaScan@0(array<string> paths);
 };