"Challenge Not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: STORAGE_LIMIT_EXCEEDED, storage limit exceeded: []"

Getting this error message when trying to check challenge. This is Automate Basic Business Processes with Process Builder module in Trailhead. I followed the steps suggested by people to check storage from system overview area. I checked that and found I have lots of storage available. Only 350 KB out of total possible of 5MB, which is only 6% of total availa storage. so 94% is free.

When I click on 350 KB and check storage usage, everything looks okay except File storage which is 112% used. Content bodies have used 99% of available storage.

Can somebody please advise what should I do. and if I have to reduce file storage how can i do that.

Thanks in advance.