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

components/arc/common/file_system.mojom

Index: components/arc/common/file_system.mojom
diff --git a/components/arc/common/file_system.mojom b/components/arc/common/file_system.mojom
index 24e4041ff8d137bc018b0cd52572b8b03c543433..d19e9235f9d18b960039615ea2ba0f0bd0055818 100644
--- a/components/arc/common/file_system.mojom
+++ b/components/arc/common/file_system.mojom
@@ -2,7 +2,7 @@
 // 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;
 
@@ -51,7 +51,7 @@ interface FileSystemHost {
   [MinVersion=3] OnDocumentChanged@0(int64 watcher_id, ChangeType type);
 };
 
-// Next method ID: 8
+// Next method ID: 9
 interface FileSystemInstance {
   // Notes about Android Documents Provider:
   //
@@ -105,6 +105,10 @@ interface FileSystemInstance {
   // 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);