Fixes #20236: Improve file naming and upload handling

[pheus]

Fixes: #20236

Summary
Silently normalize the on‑disk filename derived from image attachments to prevent nested directories and path traversal, while preserving the user‑entered name in the database/UI unchanged.

What’s changed

• In upload_to:
  • Normalize browser paths (e.g. C:\fakepath\photo.jpg) and extract the real uploaded extension.
  • Use the user‑entered name when present; sanitize via Django’s filename utility.
  • Prefix with {object_type.model}_{object_id} and validate the final relative path with validate_file_name(..., allow_relative_path=True).
  • Preserve only known image extensions (bmp, gif, jpeg, jpg, png, webp); otherwise save without an extension.
• (Hardening) Guard filename parsing to avoid IndexError if older files don’t follow the expected underscore pattern.

Behavior before

• Supplying slashes in the name could create subdirectories and trigger IndexError in code that assumes a fixed filename layout.

Behavior after

• Files are always stored under image-attachments/ with a flat, safe basename.
• The value shown/stored in the name field is not altered; only the on‑disk filename is normalized.

Notes for reviewers

• This follows the maintainer guidance to silently normalize rather than reject slashes.
• No user‑visible changes to the name field or forms.

[jnovinger]

Hey, @pheus , this looks great and achieves exactly what the original issue and discussion required. I do have one ask, though, which is to include some minimal testing of ImageAttachment.filename and image_upload to establish future expectations.

Having said that, I think it's important that we ship this in the upcoming v4.4.1 release coming next Tuesday. So, if there's not enough time to add the tests before then, we can merge this as is and add tests as a follow up. Thoughts?

[pheus]

Thanks for the quick review and for aiming this at v4.4.1. This is much appreciated!

I’ll add a minimal test and aim to push these by Monday to leave time for review. If timing gets tight, I’m fine merging as-is and following up with a tests-only PR.

[pheus]

I’ve added minimal tests covering:

• image_upload normalization (slashes/backslashes; no nested paths) and extension handling
• ImageAttachment.filename prefix handling and legacy names (no IndexError)
• Windows-style inputs (C:\fake_path\...)

I hope this aligns with what you had in mind. Thanks for the review and guidance. And for maintaining NetBox.

[jnovinger]

Thanks, @pheus . Those are exactly the sorts of tests I had in mind.