Use `ffmetadata` format to load multiple metadata fields from a file

[weak-head]

Description

To load multiple metadata fields from a file, we can:

• Create a file in ffmetadata format.
• Use -i to load it with -map_metadata.

Example

meta.txt

;FFMETADATA1
description=This is a
multi-line description
with "quotes" and such.
artist=John Doe

ffmpeg -i input.m4a -i meta.txt -map_metadata 1 -codec copy output.m4a