I found this information on the Cisco Spark site that may be useful - please see the "Known issues" section at the bottom of the page.
I hope this helps.
Moderator for Cisco Customer Communities
We were able to upload files when we only had the free version, but now that we've upgraded to an M3 trial version, we can't upload anymore. It gets to about 99% and stalls. The message stays grey and can't be flagged or deleted. Under Files, it indicates a file, but when clicked to Preview it never opens, just continues to process. When you go back to the space later, the message has disappeared.
Is there a setting that needs to be changed? Are there known issues with enterprise accounts?
I've read the online documentation, but I haven't found anything that deals with troubleshooting.
Dianne - to add to what Kelli suggested to check out, one thing I always recommend people try is to see if you're experiencing that behavior across all platforms/devices. For example - many people use the Windows (or Mac) version of the Spark and don't even realize there is a Web-based version too (found here: www.ciscospark.com). And one nice thing of the web version is that you know it's always the latest version. Whereas if you use the Windows or Mac downloadable versions, it may not be up to date. So I would make sure they try the web version too. If it works there, chances are an update for the downloadable version might be needed (or maybe they'll like using the web version instead). Also - see if they have the same issues with the mobile version. Because it's the whole mantra of your content, anywhere, on any device. So they should be able to have the capability to upload docs via mobile client. Hope that helps!!