On the definition of the attachment repository, there is a field called "Prohibited File Types". The defines the file extensions that users may not upload to the repository.
If the value in this field begins with an exclamation point (!), these file types are allowed in the current repository. For example, a value of jpg,gif in the list denotes that files with .jpg and .gif extensions are prohibited in the repository. However, a value of !jpg,gif denotes that only files with .jpg and .gif extensions are allowed in the repository.