Tweaks for Excel to Markdown conversion

[cebtenzzre]

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There are a few changes here that we didn't have time to discuss in the previous PR:

• _Underlines_ seem to be recognized by Llama 3, so use them
• Not all spreadsheets use the first row as a header. Llama 3 seems to generalize better with no header row.
• Empty cells seem to not be well supported by Llama 3 unless there is a space in them (this matters more for spreadsheets with lots of horizontal whitespace)
• Long filenames were exceeding the width and height of the attached file cards. Probably we should be using a RowLayout and not a Row so it's easier to automatically constrain the width of the elements, but this change works too.

~~Needs a changelog entry once we decide which of these changes to keep.~~

Follow-up to #3007