In Salesforce you have 2 storage categorycategories
File Storage - This constitutes all the files / attachments that is uploaded in your org.
Data Storage - This is where your standard and custom object data resides.
Developer edition usually have very low data storage limits (5 Mb which roughly is 2.5k records (2kB -per record)). So you could delete some of the data (std / custom records) in your org and then you will be able to create new records.
You can check the storage limits by going to :
SetUp --> Administer -> Data Management -> storage Usage