This PR fixes an issue where an image mention would have its label reset
to just "Image", instead of persisting the original label, when the
prompt got submitted.

Closes #48564

Release Notes:

- agent: Fixed image mention labels by persisting the file name after
submitting the prompt
- agent: Fixed directory mentions being incorrectly parsed as files when
pasting into prompt editor

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Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>