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

components/arc/test/fake_file_system_instance.h

Index: components/arc/test/fake_file_system_instance.h
diff --git a/components/arc/test/fake_file_system_instance.h b/components/arc/test/fake_file_system_instance.h
index b852244e701f616496c89ed1ded4b6bff625f44b..0d84ebab7b0daf25b2dffdd8603d4b7f8f475344 100644
--- a/components/arc/test/fake_file_system_instance.h
+++ b/components/arc/test/fake_file_system_instance.h
@@ -29,6 +29,7 @@ namespace arc {
 //
 // Content URL based functions are:
 // - GetFileSize()
+// - GetMimeType()
 // - OpenFileToRead()
 // Fake files for those functions can be set up by AddFile().
 //
@@ -56,10 +57,16 @@ class FakeFileSystemInstance : public mojom::FileSystemInstance {
     // The content of a file.
     std::string content;
 
+    // The MIME type of a file.
+    std::string mime_type;
+
     // Whether this file is seekable or not.
     Seekable seekable;
 
-    File(const std::string& url, const std::string& content, Seekable seekable);
+    File(const std::string& url,
+         const std::string& content,
+         const std::string& mime_type,
+         Seekable seekable);
     File(const File& that);
     ~File();
   };
@@ -130,6 +137,8 @@ class FakeFileSystemInstance : public mojom::FileSystemInstance {
                    const GetDocumentCallback& callback) override;
   void GetFileSize(const std::string& url,
                    const GetFileSizeCallback& callback) override;
+  void GetMimeType(const std::string& url,
+                   const GetMimeTypeCallback& callback) override;
   void Init(mojom::FileSystemHostPtr host) override;
   void OpenFileToRead(const std::string& url,
                       const OpenFileToReadCallback& callback) override;