We have a case open to consider this feature for a future release. Please up-vote this question (not this answer) to show your support for adding this feature.
Until this is implemented as a proper feature or plugin, you can use the following BugMonkey customization:

name: Warn about emails with missing attachments
description: Warns if your email says "attach" but you haven't attached anything
author: Daniel LeCheminant
version: 1.0.0.0
js:
var sWarning =
"Your message mentions attachments, " +
"but you haven't attached anything!\n\n" +
"Do you still want to send the message?";
if(!window.clickBugSubmit) return;
window.clickBugSubmit = (function(fnOrig) {
return function(e, elForm, fXMLSubmit, sValue, bOK) {
if(bOK) {
var fHasAttach = $("#files_list div").length;
var sEmailText = $("#bugviewContainerEdit .emailHeader").siblings()
.find("textarea").val();
var fSaysAttach = /\battach/i.test(sEmailText);
if(fSaysAttach && !fHasAttach && !confirm(sWarning)) {
return cancel(e);
}
}
return fnOrig.apply(this, arguments);
};
})(window.clickBugSubmit);
If you have BugMonkey 2.0 and an administrator has enabled importing from fogbugz.stackexchange.com, you should be able to send the above customization directly to your fobugz. There will be a button to do so at the top left of the code box above.