From what I understand, Hillary Clinton and her supporters demanded that Bernie Sanders release his tax return, suggesting that he was refusing to do so and was hiding something, even though the demand was made before April 15th and the tax return may not have even been written yet. Bernie promptly complied with the demand and it apparently contained nothing interesting after all.
This seems to just make Hillary Clinton look bad as it is an opportunity to contrast Bernie's (relatively) mundane Senator's salary and Social Security benefit to her great wealth, and his apparent transparency with her steadfast refusal to release her Wall Street speech transcripts, which Bernie and his supporters have been demanding for months, and the seeming motivation for the tax return demand.
Was this attack strategy successful and am I just not aware of it? What did they believe they would find in the Sanders tax return? Why did they think this was a good strategy to attack Bernie?