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The file name to validate. The file name must be a valid file
- It must only contain letters, numbers, and the following special
characters: `@`, `_`, `+`, `/` `$`, `.`, `-`, or '~'.
- It must be less than 256 characters.
- It must not contain `/./`, `/../`, or `//`.
- It must not end in `/`.
- All spaces must be in the middle of a directory or file name.
An error string if the file name is invalid. '' is returned if the file
name is valid.