When writing a Hotmail email, I can't attach files of any type. This happens only on my Macbook Pro, not my iMac. the last time I was successfully able to attach files to a Hotmail email on my Macbook Pro was on the 1 December 2011. Since then, when I click on Attachment, a go to the folder where the document is, click on it and then only get the option to OPEN, rather than CHOOSE. When I click on OPEN, after a while I get an error message "Attachment Id has bad file format".
Try upgrading your Silverlight. It has been known if the wrong version is installed Hotmail attachments will fail to work. If that doesn't work, you may want to look for httpdmail for Mac OS X Mail which supports Hotmail under Mac OS X Mail.