Do not handle a file as application/zip when the extension is not .zip.

chrome/browser/extensions/api/file_handlers/mime_util.cc

Index: chrome/browser/extensions/api/file_handlers/mime_util.cc
diff --git a/chrome/browser/extensions/api/file_handlers/mime_util.cc b/chrome/browser/extensions/api/file_handlers/mime_util.cc
index 74b622c26bdf7ec03b792d0d409553995be20d29..44198112627cb1180427c8150d86d8414de3fb08 100644
--- a/chrome/browser/extensions/api/file_handlers/mime_util.cc
+++ b/chrome/browser/extensions/api/file_handlers/mime_util.cc
@@ -37,6 +37,12 @@ void SniffMimeType(const base::FilePath& local_path, std::string* result) {
                        net::FilePathToFileURL(local_path),
                        std::string(),  // type_hint (passes no hint)
                        result);
+
+    // If the extension is not .zip, do not handle the file as application/zip
+    // since it might be unknown internally zipped file.
+    if (*result == "application/zip" &&
+        local_path.Extension() != FILE_PATH_LITERAL(".zip"))

[benwells, 2015/04/14 21:57:22]

The logic of this flow is, I think:
- return the mime type from the extension, if it isn't empty, otherwise
- return the sniffed mime type from the contents, except if the extension is not
zip and the mime type is, in which case return the applicaiton/octet-stream

This doesn't seem that clean. In particular:
- if there are other valid application/zip extensions this now feels
inconsistent
- but, there shouldn't be any need to check the extension as .zip files
shouldn't end up being sniffed anyway

Instead of doing this you could add something to the handler of this
(OnSniffMimeTYpeForNativeLocalPathCompleted) which checks the returned
mime_type:
  if (*mime_type == "application/zip") {
    callback.Run("application/octet-stream");
    return;
  }

  callback.Run(*mime_type)

THat way you don't redundantly check the extension again.

But, maybe I'm missing some reason to do it this way...

[yawano, 2015/04/15 03:41:34]

Moving to OnSniffMimeTypeForNativeLocalPathCompleted seems to be better. I moved
it.

+      result->assign("application/octet-stream");

[benwells, 2015/04/14 21:57:22]

Why is the default application/octet-stream? Is that the default we always use?

[yawano, 2015/04/15 03:41:34]

I think there is no default mime type. The default value of mime type is empty
string.

https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/ext...

The reason that we selected application/octet-stream is that generic mime type
should be less confusing.

   }
 }