FIX: Converter Mixed Media Types

[jbolor21]

Description

In the converter GUI panel, when we use the converters to change media types (ie add text to image: text -> image or PDFconverter takes text -> binary file output) this PR now shows the converted file as a file rather than text and lets you open the converted file as well!

• convertersApi.previewConversion now returns converted_value_data_type alongside converted_value, so the frontend knows what kind of artifact came back.
• PieceConversion gains a convertedDataType field plus utility helpers (isPathDataType, dataTypeToAttachmentKind, buildMediaUrl, basenameFromValue) in converterTypes.ts for consistently turning backend paths into media URLs and display names.

Tests and Documentation

Ran existing tests & added new tests for testing the new "open" file button loads and renders a file