Fixes #309 - FolderFileStorage doesn't overwrite existing content

[niemyjski]

File.OpenWrite(fullPath)

• Opens the file if it exists.
• Does not truncate the file; it only allows writing from the current position.
• If the file does not exist, it creates a new file.

FileStream with FileMode.Create

• Creates a new file or overwrites an existing file.
• Truncates the file to zero length if it already exists.
• Opens the file for writing.