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As a server administrator, you can use Central Administration to restrict certain kinds of files from being uploaded or retrieved, based on the file extension. For example, a file with the .exe file extension can potentially contain code that runs on client computers when the file is downloaded. If files with the .exe file extension are blocked, users can neither upload nor download a file with the .exe extension. By blocking this file type, potentially dangerous content in the .exe file cannot be downloaded and run on client computers. This feature does not prevent all exploits based on file types, nor is it designed to do so.

 Note   It is helpful for site owners and other users of a site to know what file types are being blocked so that they understand why they cannot upload certain kinds of files. If you are unsure what file types are being blocked, talk to your server administrator.

By default, several standard file extensions are blocked, including any file extensions that are treated as executable files by Windows Explorer. Files with curly braces { or } are also automatically blocked. The file extensions blocked by default are shown in the following table.

Server administrators can use Central Administration to choose which file extensions to block for the entire server or server farm. Because the list of blocked file types is maintained by file extension, any file that uses a file extension on the list cannot be uploaded or downloaded, regardless of the file's intended use. For example, if .asp is on the list of extensions to block, the feature blocks all .asp files on the server, even if they are used to support Web site features on another server in the server farm. If a file ends in a period (.), the preceding characters are checked against the list of blocked file extensions as well. For example, if .exe is on the list of blocked file extensions, a file called "filename.exe" is also blocked. The following list shows different ways of representing the same file, all of which are blocked if the .hta extension is on the list of blocked file extensions.

• filename.hta
• filename.hta.
• filename.hta.{3050F4D8-98B5-11CF-BB82-00AA00BDCE0B}
• filename.hta::$DATA

You can determine which files are blocked for Web sites on your servers by modifying the list of blocked file extensions in Central Administration. You can block additional file extensions (up to 1,024 file types) by adding them to the list in the Central Administration pages, or you can remove a block by deleting the file extension from the list. When you change the list of file extensions, the change affects both new files being added to a Web site and files already posted to a Web site. For example, if a document library  contains a .doc file, and you add the .doc file extension to the list of blocked file extensions, users will no longer be able to open the .doc file in the document library. Users will be able to rename or delete a file with a blocked file extension but will not be able to perform any other actions.