You can set a default for file types allowed in your settings, but sometimes you may want to override that for a particular field. You can do this by placing a hidden input field like this inside the form where the file is being submitted:

<input type="hidden" name="attachfieldtypes" value="exe,zip,gz">

Change "attachfieldtypes" to be the name of your attachment field followed by types. For example, if your field is named myattach you'd use

<input type="hidden" name="myattachtypes" value="exe,zip,gz">

Replace exe,zip,gz with the file extensions you want to allow.