It is a configuration option, if you want to allow mixed case files and folders, you can add this to your user.config file:
<add key="ForceLowerCaseForFolderCreation" value="false" />
<add key="ForceLowerCaseForUploadedFiles" value="false" />
The reason we force lower case by default is because mojoPortal can run on Linux. On Windows urls are not case sensitive because the file system is not case sensitive, but on Linux both the file system and urls are case sensitive so one mistake in capitalization can cause a broken url and forcing lower case ensures a convention that will prevent that.
If you are 100% sure you will never move your mojoPortal site to Linux or host the files on a Linux server it is fine to allow mixed case.