[Ansansan]

Fixes #1853

printf '' | llm -m mock -a - hi (and -a with an empty file) crashed with puremagic's PureValueError: Input was empty. For empty input puremagic raises a ValueError subclass rather than PureError (with puremagic 2.2.0, and PureValueError isn't exported at the top level), so it got past both mimetype_from_string and mimetype_from_path.

Both helpers now catch (puremagic.PureError, ValueError) and return None, so the CLI shows its usual usage error:

Error: Invalid value for '-a' / '--attachment': Could not determine mimetype of stdin

Tests:

• test_prompt_attachment_empty[stdin] and [file]: exit code 2, the error message, and the model never called
• test_mimetype_of_empty_content: both helpers return None for empty content

All three failed before the change with the traceback from the issue. The full suite passes (1401), and black --check, mypy llm, ruff check and cog --check are clean.

Prepared with Claude Code.

[chrikrah]

This fixes #1853. After a rebase on main, it can be merged from my side. #1857 landed after your base and also adds tests at the end of tests/test_attachments.py, so that file conflicts. I kept both blocks. Then empty stdin, an empty file and an empty pipe all give the -a usage error, exit 2. The three new tests fail with the old utils.py.

# Python 3.12.3, puremagic 2.2.0; base b9786a2daa, head e91985abbb, main 39d325492b; PYTHONPATH=<exported tree>
printf '' | python -m llm -m nosuchmodel -a - hi
  (base) -> puremagic.main.PureValueError: Input was empty   (exit 1)
  (head) -> Error: Invalid value for '-a' / '--attachment': Could not determine mimetype of stdin   (exit 2)
python -m llm -m nosuchmodel -a empty.bin hi   (head) -> ... Could not determine mimetype of empty.bin   (exit 2)
pytest tests/test_attachments.py -q   (base) -> 13 passed
pytest tests/test_attachments.py -q   (head) -> 16 passed
pytest tests/test_attachments.py -q   (head, utils.py reverted) -> 3 failed, 13 passed
# git merge-tree --write-tree origin/main e91985abbb, conflict resolved by keeping both test blocks
pytest tests/test_attachments.py -q   (merged) -> 18 passed
python -m llm -m nosuchmodel -a <(printf '') hi   (merged) -> ... Could not determine mimetype of /proc/self/fd/13   (exit 2)

non-blocking: the except in mimetype_from_path has no comment on why ValueError is there. Copy the one from mimetype_from_string?