fix(chat): avoid duplicate pasted image attachments

[LugMuad]

Summary

Fix duplicate image attachments when pasting an image into the chat composer.

Some browsers expose the same pasted image through both DataTransfer.files
and DataTransfer.items. The previous implementation collected files from both
sources, which could attach the same clipboard image twice because the resulting
File objects may have different metadata.

Prefer DataTransfer.files when available and use DataTransfer.items only as
a fallback.

Tested

• Pasting a clipboard image now creates a single attachment.
• Regular file attachment behavior is unchanged.