How to change default value of term_vector for all fields of type string

Hi,
is there a way to change the default value of "term_vector" (default "no")
for field type "string"? I would like ES to continue generating mappings
dynamically (I can't define my mapping in advance) but I want all the
"string" fields to be created with "term_vector" =
"with_positions_offsets".
Thank you,
Mik.

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