Add http.fileupload.max.file.count configuration and refactor file upload handling

[marevol]

This update introduces a new configuration property http.fileupload.max.file.count to limit the maximum number of files that can be uploaded in a single request. Additionally, the file upload handling logic has been refactored to improve clarity and maintainability. Key changes include:

• Added support for setting and retrieving the max file count in the configuration.
• Updated FessMultipartRequestHandler to enforce the file count limit during upload processing.