Epsom Salts is a Hyperosmotic saline type laxative. This means that the magnesia acts to pull a salt or saline solution into the intestines from the blood. This excessive fluid then provides a flushing or laxative action within the intestines. There is an inherent loss of salt from this process.
You will not find medical evidence concerning magnesium sulphate and dilation of the bile duct because conventional medicine doesn't use it. They use other drugs given by intravenous injection, which are not practical for home use.
The use of Epson Salts has never been in doubt and its effects when taken orally are listed on the GIT database: