Check only final extension in DeepScanningDialogDelegate

The previous implementation would result in the entire extension being
used, so atypical ones like "foo.csv.gz" would check if ".csv.gz" had
a corresponding mimetype instead of just checking ".gz".

Bug: 1069347
Change-Id: Ieefb8f4726d4a268e3bc312592a1c53060007210
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2158973Reviewed-by: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761045}

chrome/browser/safe_browsing/cloud_content_scanning/deep_scanning_dialog_delegate.cc

@@ -137,8 +137,15 @@ bool ShouldShowWarning(const DlpDeepScanningVerdict& verdict) {
 std::string GetFileMimeType(base::FilePath path) {
   // TODO(crbug.com/1013252): Obtain a more accurate MimeType by parsing the
   // file content.
+  base::FilePath::StringType ext = path.FinalExtension();
+  if (ext.empty())
+    return "";
+
+  if (ext[0] == FILE_PATH_LITERAL('.'))
+    ext = ext.substr(1);
+
   std::string mime_type;
-  net::GetMimeTypeFromFile(path, &mime_type);
+  net::GetMimeTypeFromExtension(ext, &mime_type);
   return mime_type;
 }