From your Safari menu bar click Safari > Preferences then select the Privacy tab.
Click Details. Type Microsoft in the search field top right corner of the window then click Remove All > Done.
Do that one more time. Type in Hotmail
Now from the menu bar click Safari > Empty Cache.
Quit then relaunch Safari. Try attaching files.
In case you are not aware, you can avoid using web based email (via a browser) to access your Hotmail account.
Launch Mac Mail from the Dock
From the Mail menu bar click Mail > Preferences then select the Accounts tab then the Account Information tab.
Click +
Enter your name, Hotmail user name and password, click Continue or Create.
If you want to start a new mail message just click the Mail icon in your Dock. If you have unread messages, you'll see a red badge on the Mail icon.