fix: do not append media dimensions to non-embeddable links

What does this MR do and why?

This MR fixes an issue where media dimensions ({width=... height=...}) were incorrectly appended to non-embeddable media links when pasting files into the plain text editor.

By adding a guard clause (if (!md.startsWith('!')) return md;) in transformMediaMarkdown inside app/assets/javascripts/dropzone_input.js, we ensure that only properly embeddable media formats get the dimension attributes. This prevents stray brackets from rendering as literal text for unsupported video formats like .mkv or .bin.

Closes #632782

Screenshots or screen recordings

No visual UI changes, but the generated markdown text in the textarea no longer contains stray dimensions.

How to set up and validate locally

1. Open any issue or merge request to view the plain text editor.
2. Copy and paste a non-embeddable video file (e.g., a .mkv file or any dummy file with a .bin extension) into the textarea.
3. Verify that the generated markdown is simply [filename.mkv](/url/to/file) and does not have {width=... height=...} appended.

MR acceptance checklist

This MR has been evaluated against the MR acceptance checklist.