Converting from an audio block with a formatted caption to a file block results in an incorrect file link

[talldan]

Describe the bug
When converting from an audio block that has a formatted caption (bold/underlined/strikethrough formating applied) to a file block, the caption is expected to be displayed as the link to the file. However, the conversion process is not correctly removing the applied formatting, causing [object Object] to be displayed.

File block is a new block being introduced in #7622

To Reproduce
Steps to reproduce the behavior:

1. Create an audio block
2. Add a caption to the audio block with formatted text (e.g. bold, underline, strikethrough, italic)
3. Convert the audio block to a file block
4. Note that the file link contains [object Object] where formatting was applied

Expected behavior
Any formatting should be removed and purely the text of the caption should be shown as the file link

Screenshots

[sarahmonster]

This issue also exists when converting an image to a file:

[jorgefilipecosta]

This bug was solved during the changes to the formatting API.