improper use of file_validate_extensions on upload

Trying to import files as any user other than UID 1 (that does have permission to import taxonomies) results in a validation error:

"The selected file taxonomy_artist_type.xml could not be uploaded. Only files with the following extensions are allowed: upload_file."

This is because the form sets file_validate_extensions as the validation function, but does not correctly pass in extensions that are actually valid.