Fix dropping of WebP images to the Finder.

mime_util does not take extensions with a leading dot, and if you
pass in an extension with a leading dot, you will not successfully
look up a mime type.

BUG=84003
TEST=as in bug

Change-Id: I37d9b0946df61a557b3423ae8a13ae447ed6c7c1
Reviewed-on: https://chromium-review.googlesource.com/c/1493473
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636446}

content/browser/web_contents/web_drag_source_mac.mm

@@ -364,7 +364,7 @@ - (void)fillPasteboard {
           dropData_->GetSafeFilenameForImageFileContents();
       if (suggestedFilename) {
         downloadFileName_ = std::move(*suggestedFilename);
-        net::GetMimeTypeFromExtension(downloadFileName_.Extension(), &mimeType);
+        net::GetMimeTypeFromFile(downloadFileName_, &mimeType);
       }
     } else {
       base::string16 mimeType16;

net/base/mime_util.cc

@@ -303,6 +303,9 @@ bool MimeUtil::GetMimeTypeFromExtensionHelper(
     const base::FilePath::StringType& ext,
     bool include_platform_types,
     string* result) const {
+  DCHECK(ext.empty() || ext[0] != '.')
+      << "extension passed in must not include leading dot";
+
   // Avoids crash when unable to handle a long file path. See crbug.com/48733.
   const unsigned kMaxFilePathSize = 65536;
   if (ext.length() > kMaxFilePathSize)