A user is uploading a document in the Appian environment and wants to restrict the...

 The best way to restrict the uploading of documents without extensions at the environment level is to enable the File Upload > Block files without an extension option under the Administration Console. This option will prevent users from uploading files that do not have a valid file extension, such as .docx, .pdf, or .png. This can help to avoid security risks and compatibility issues with unsupported file types. Therefore, the correct answer is A.

References:

[File Upload Settings]

[File Extensions]