A ten-foot long snake made people run here and there.
Enclosed with this letter were a signed affidavit and a carbon copy of his request to our main office.

in the first sentence ten-foot work as an adjective. adjectives have no plural. feet is a plural.
in the second sentence the letter was attached with two papers, an affidavit and a carbon copy of his request. Hence was should be replaced by were, the plural form
Regards.