Avoid file IO on UI thread by moving more save-page-as stuff to the FILE thread.

base/nix/mime_util_xdg.h

 // Copyright (c) 2011 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.
 
 #ifndef BASE_NIX_MIME_UTIL_XDG_H_
 #define BASE_NIX_MIME_UTIL_XDG_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "build/build_config.h"
 
 namespace base {
 
 class FilePath;
 
 namespace nix {
 
-// Gets the mime type for a file based on its filename. The file path does not
-// have to exist. Please note because it doesn't touch the disk, this does not
-// work for directories.
+// Gets the mime type for a file at |filepath|.
+//
+// The mime type is calculated based only on the file name of |filepath|.  In
+// particular |filepath| will not be touched on disk and |filepath| doesn't even
+// have to exist.  This means that the function does not work for directories
+// (i.e. |filepath| is assumed to be a path to a file).
+//
+// Note that this function might need to read from disk the mime-types data
+// provided by the OS.  Therefore this function should not be called from
+// threads that disallow IO via base::ThreadRestrictions::SetIOAllowed(false).
+//
 // If the mime type is unknown, this will return application/octet-stream.
 BASE_EXPORT std::string GetFileMimeType(const FilePath& filepath);
 
 // Get the mime type for a byte vector.
 BASE_EXPORT std::string GetDataMimeType(const std::string& data);
 
 }  // namespace nix
 }  // namespace base
 
 #endif  // BASE_NIX_MIME_UTIL_XDG_H_