Merge pull request #3681 from pallets/fix-typo

fix typo

docs/patterns/fileuploads.rst

@@ -182,7 +182,7 @@ construction of progress bar.
 Because the common pattern for file uploads exists almost unchanged in all
 applications dealing with uploads, there is also a Flask extension called
 `Flask-Uploads`_ that implements a full fledged upload mechanism that
-allows controlling with file extensions are allowed to be uploaded.
+allows controlling which file extensions are allowed to be uploaded.
 
 .. _jQuery: https://jquery.com/
 .. _Flask-Uploads: https://flask-uploads.readthedocs.io/en/latest/