Error 2550 while trying to add sound

I'm attempting to add an MP3 voiceover file to my Prezi following the directions: https://prezi.zendesk.com/entries/225..., but I keep receiving the following error message:

File type not supported
We support Images (JPEG, PNG, GIF), Videos (MPEG, AVI, WMV), Presentations (PPT, PPTX) and PDF.
Technical details:
id: 2550 the reason is: RequestS3UploadToken

I am using the free version of Prezi for educators. What, if anything, am I doing wrong, and how might I fix it? Thanks.