Best bet would be to just set the property/attribute value you want in the transformation. On the
Form tab of a
CMS.File you can set the
"file name" property and/or description. Then in your transformation you can simply set the
alt/title="<%# Eval("FileName") %>"
and it will render what the user has entered.
If you want to force users to enter a value, simply make the field required in the CMS.File document type.
Best of luck!