Yes, this is possible. You will need to add the filetypes that you want to allow. Here's how. Open /conf/_admin.php and find this line:

$fileupload_allowedtypes = ' jpg gif png ';

Add the filetypes you want like this:

$fileupload_allowedtypes = ' jpg gif png txt doc rtf';

In b2evolution 1.6 (not released yet) you'll be able to do this in the backoffice settings rather than in a config file.