For reference, here is a basic ASCII table. The sort follows the values in this table: https://commons.wikimedia.org/wiki/File:ASCII-Table-wide.svg
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This was an expected result from a technical point of view, but it's unexpected from a user point of view. We should probably update the implementation to make it work like expected by users. If that's ASCII sort for all fields, it's probably a change we want to discuss on the forum. If that ASCII sort is specific to attachment names (and it wasn't already the result of a discussion), we can update it without asking everyone IMO.
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