For everyone getting the "encoding failed" this is what I learned talking with Maverick about the problems I got:
1) Your link has to be directly for the download, it can't lead you to a page where you have to click somewhere to download it.
My first try was with the link: https://www.dropbox.com/s/nhwrhd47saore3a/20131124_2122_koth_coalplant_b2a_w2r_1st.dem
But if you click it, you still have to click in "Download" to get the file. But the "rendering bot" doesn't know that, so he "thinks" he is getting a really small .dem file.
So I sent this link: https://dl.dropboxusercontent.com/s/nhwrhd47saore3a/20131124_2122_koth_coalplant_b2a_w2r_1st.dem
And it worked!
2) The url needs to end in .dem
When I was getting the second link, Dropbox gave me actually the link: https://dl.dropboxusercontent.com/s/nhwrhd47saore3a/20131124_2122_koth_coalplant_b2a_w2r_1st.dem?dl=1&token_hash=AAFHY6Rgs-ykYyPnhaSkBk0UmlZxz_6GQe9jxljAcIY8hw
But when you try to send it, the site thinks this is not a .dem file, so I removed everything after .dem and tried again. Worked.