I’d been getting quite frustrated by the File Uploader tool ‘removing’ an uploaded audio file’s extension when using Chrome or Samsung Internet on my Galaxy S8; it would just show up as the file name in the database without ‘.mp3’ at the end. So the Circle Music player wouldn’t know what to do with it. Even the MIME-type plugins weren’t working - the system was not passing the .mp3 through.
After many emails to the very helpful Bubble Support team, and many trials across many friends, it seems like it’s a cross between Samsung/Android and which options the system determines to show you. If you use the red-circled ‘Files’ button it won’t work, but if you use the green-circled ‘My Files’ the extension will be kept. This issue is not present on the S6, RealMe, Pixels or iOS - so random. In a random twist, though, if you use Firefox - the ‘Files’ button works!
The chances of anyone else needing this are small, but hey