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..6f0005fcc94afc5aa96a0b2af275c7d5269b27ac 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 +105,11 @@ 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. When an error occurs, returns null value (not to be confused with the
+  // empty string).

[mtomasz, 2017/06/05 04:29:55]

optional: How are directories handled? On Android it's
"vnd.android.document/directory" which may not make sense on Chrome OS side. Do
we need to care about it?

[hashimoto, 2017/06/05 04:55:37]

Android-side impl just calls ContentResolver.getType() with the given URL, so it
returns what ContentReoslver.getType() returns for a directory when it happens.

IIUC no Chrome OS code should try to get MIME type of a directory (i.e.
is-directory check should be done before calling
GetNonNativeLocalPathMimeType()) so I think it's OK to leave the behavior
undocumented.
WDYT?

[mtomasz, 2017/06/05 05:01:06]

SGTM, thanks for explanation.

+  [MinVersion=4] GetMimeType@8(string url) => (string? mime_type);
+
   // Establishes full-duplex communication with the host.
   [MinVersion=3] Init@5(FileSystemHost host_ptr);