Recently, I discovered I wasn't able to create a new SharePoint 2010 site using the out-of-box (OOB) team site template in my portal Web application. However, when I tried to create a site using the blank site template, the site was created successfully.
After digging deeper into the odd behavior I was experiencing, I discovered that I had previously blocked some file types for my portal Web application and one of those file types that I had blocked was the .png file type. Knowing a blocked file type prevents a file from being uploaded, and even downloaded, in SharePoint I decided to check to try to determine which file types are used when creating a new site using the team site template.
After opening up a site that I had previously created using the Team Site template I noticed the default picture that is automatically populated on the top right side of the team sites when using the OOB site template.
Remembering I had blocked the .png file type I decided to allow the .png file type for my portal Web application. I removed it from the list of blocked file types and then tried again to create a new site using the team site template and sure enough it was created successfully.
With this discovery, I think it is important that everyone understands that blocked file types are not only applicable to libraries, but they are also applicable to both OOB and custom site templates as well as features added to SharePoint. With this said, it is important that you are very careful when deciding what file types you are blocking per web application being the file types are also blocked when using SharePoint templates and features.