This happened to me and another user too (new Macbook and latest version of OSX).
Tried to upload the GPX file from motocaching to MyRoute App and it said that it's not a GPX file.
Finally we found out what was wrong and how to fix it:
The problem was that the file xxxx.gpx was renamed to xxxx.gpx.xml when downloading it.
Tha difficult part was that you can't fix it by just normally renaming the file in finder to xxxx.gpx. That changes the file name but it doesn't change the xml file type association!
Solution : Right click your file in the Finder and select 'Get Info'. Now change the file name and extension in the info window (i.e. remove the .xml). At least this worked for me and my friend.
ps. No idea why the xml was added to the name of the downloaded file... It doesn't happen on my 15" MacBook Pro that I use for work but when I recently bought another 13" MacBook Pro, I noticed that it added the xml at the end of the name.