LOL!!! So about two seconds after posting all of that ^ I finally figured it out!
Some things are definitely not as intuitive on the Mac! Here are the steps in the event anyone else can benefit (this is assuming you've saved the tcx file, which the Mac automatically coverts to a text file upon save):
1. Right click the file and choose "Get Info"
2. On the menu that pops up, click on "Name & Extension"
3. Then remove the .txt part of the file name
4. You'll then get the following prompt: Are you sure you want to change the extension from “.txt” to “.tcx”?
5. Click "Use .tcx"
So basically what chumley was saying all along but instead of just right clicking on the file name like in PC, you have to go to the 'Get Info' section first.
I tested it by uploading to HAZ and it worked. Since I already uploaded the route yesterday from my PC, I deleted it; but the upload was definitely successful.