Switch to nsIFileProtocolHandler.getURLSpecFromActualFile() for compatibility with TB 94 beta.

addon/content/sendMessage.js

@@ -34,7 +34,7 @@ function locationForContent(content) {
   var fileHandler = Services.io
     .getProtocolHandler("file")
     .QueryInterface(Ci.nsIFileProtocolHandler);
-  return fileHandler.getURLSpecFromFile(file);
+  return fileHandler.getURLSpecFromActualFile(file);
 }
 
 function convertAttachment(pep) {