Starting a format string with '\f<Type> ' allows to set a different
format string type than the default. Available ones are
- T: A template file containing the actual format string
"\fT ~/.templates/booru.txt
- E: An arbitrary Python expression
"\fE title.upper().replace(' ', '-')"
- M: Name of a Python module followed by a function name inside it.
This function gets called with the current metadata dict as
argument and should return a string.
"\fM my_module:generate_text"
'\f' was chosen since it is highly unlikely that a regular format string
would start with it, but that character could be changed to for example
'\\' or '/' etc.
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