Some websites block `--attachment url` requests

[ar-jan]

llm -m venice/mistral-31-24b -a https://upload.wikimedia.org/wikipedia/commons/a/a9/Corvus_corone_-near_Canford_Cliffs%2C_Poole%2C_England-8.jpg "Identify"
Error: Invalid value for '-a' / '--attachment': Client error '403 Forbidden' for url 'https://upload.wikimedia.org/wikipedia/commons/a/a9/Corvus_corone_-near_Canford_Cliffs%2C_Poole%2C_England-8.jpg'

Seems due to usage of httpx.head(value) in llm's resolve_attachment().