In Spain, is income from mortgage interest considered income or capital gains?

I’m advising someone who is a tax resident of Spain interest in providing a mortgage to someone in the U.S., and trying to get an idea of how the interest from this mortgage would be taxed there.

From https://home.kpmg.com/xx/en/home/insights/2011/12/spain-income-tax.html,

Dividends, interest, and rental income

As a general rule, investment income, such as dividends and interest arising from bank deposits, any gains on sales of shares, and so on, obtained by a Spanish tax resident will be taxed at a rate of 19 percent for amounts up to EUR 6,000, 21 percent for income in an amount between EUR 6,000 and 50,000 and 23 percent for amounts exceeding EUR 50,000.

Property rental income is included in the general income, and therefore is taxable at general progressive rates. The expenses related to such income, such as interest paid on loans linked to the acquisition of the property, local taxes, depreciation, and so on, are deductible within certain limits. In the case of rental income derived from housing, once the net income is calculated, only a 40 percent of said figure should be included in the taxable base and consequently subject to taxation.

So interest from a bank deposit is taxed at a low rate, but property rental is taxed as ordinary income (up to 45%).

There is no specific mention, though, of income from the interest obtained by providing a mortgage to another person. How would this be taxed?