This handler will output to a file using an optional mode (default is a, e.g. append). The file will grow indefinitely. It uses a buffer for writing to file. Logs can be manually flushed with fileHandler.flush(). Log messages with a log level greater than error are immediately flushed. Logs are also flushed on process completion.

Behavior of the log modes is as follows:

• 'a' - Default mode. Appends new log messages to the end of an existing log file, or create a new log file if none exists.
• 'w' - Upon creation of the handler, any existing log file will be removed and a new one created.
• 'x' - This will create a new log file and throw an error if one already exists.

This handler requires --allow-write permission on the log file.