TYPO3 Core

Steps to reproduce:

1. In the old EM, click on the "download" link of a (local) extension and save the T3X file.
2. Go to the "Import" section of the old EM.
3. Import the downloaded T3X file.

Expected result:
The extension gets imported:

Actual result:
Wrong file format. No data recognized, Error: MD5 mismatch. Maybe the
extension file was downloaded and saved as a text file by the browser and
thereby corrupted!? (Always select "All" filetype when saving extensions).

The problem occurs with the download link in the list and detail view of the old EM as well as the new EM. I've been able to reproduce the problem with Chrome and Firefox.

The created T3X has three additional bytes at the beginning (compared to a working T3X): ef bb ef

If I delete the three bytes, I can import the T3X again.

I've attached one of the corrupt T3X files.
(issue imported from #M17430)