feat: add image drag-drop and paste support to codegen chat

[rlueder]

Summary

• Users can now drag-drop or paste images (PNG, JPG, GIF, WebP, SVG, BMP) into the Claude Code chat input
• Images appear as thumbnails in the attachment list, matching how other attachments (files, commits) work
• On submit, images are decoded from base64, written to temp files, and Claude is instructed to use its Read tool to view them (since the SDK is text-only)
• Includes a 5MB size limit per image with a toast notification for oversized files

How it works

Since claude_agent_sdk_rs only exposes client.query(&str) (text-only), images can't be sent as native multimodal content blocks. Instead:

1. Frontend: Images are converted to base64 and stored in the attachment service
2. Backend: On submit, base64 is decoded, written to a temp file at {tmp}/gitbutler-images/{uuid}.{ext}, and the message tells Claude the file path so it can use its Read tool to view it

Screenshot

Example: user pastes a screenshot and asks Claude to fix a UI issue based on the image

Changes

File	Change
apps/desktop/src/lib/codegen/types.ts	Add image variant to PromptAttachment
packages/ui/src/lib/index.ts	Export FileUploadPlugin
apps/desktop/src/components/codegen/CodegenInput.svelte	Wire FileUpload plugin, implement handleFileDrop
apps/desktop/src/components/codegen/AttachmentList.svelte	Render image thumbnails
crates/but-claude/src/lib.rs	Add Image(ImageAttachment) to enum
crates/but-claude/src/legacy.rs	Add Image variant + From impl
crates/but-claude/src/session.rs	Handle image in validate_attachment and format_message_with_attachments
Cargo.toml / crates/but-claude/Cargo.toml	Add base64 dependency

Test plan

• Drag a PNG/JPG from Finder into the chat input — appears as thumbnail in attachment list
• Copy an image (e.g., Cmd+Shift+4 screenshot), paste into chat — appears as attachment
• Send a message with an image attached — Claude can view the image via Read tool
• Click X on image attachment to remove it
• Try a >5MB image — shows error toast
• Verify existing attachment types (file, commit, lines, directory) still work

🤖 Generated with Claude Code

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[krlvi]

hey! thanks for the PR - do we know if this works on various platforms? I remember there being a drag and drop of external files on mac

[krlvi]

we might wanna use the "tempfile" crate for this

[rlueder]

hey! thanks for the PR - do we know if this works on various platforms? I remember there being a drag and drop of external files on mac

Works well on mac and Linux, I have a Windows machine I can give it a go and will report back.