[FEATURE]: Allow arbitrary files to be attached as tool-accessible context

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I sometimes need to attach files that OpenCode cannot send directly to the selected model, such as PDFs, Office documents, archives, logs, or files with uncommon extensions.

I am not expecting every model to understand every file format. However, OpenCode Desktop should allow any local file to be attached and make its path and metadata available to the Agent, so it can inspect the file through tools, Skills, plugins, or MCP servers when possible.

The UI should clearly show whether a file was sent directly to the model, extracted as text, made available through tools, or could not be processed. It should never silently accept a file while leaving the user unsure whether the Agent can access it.

This is broader than Office document parsing in #27689: the main request is a predictable fallback that lets arbitrary files remain available to the Agent even when their format is not supported natively.