Fix explicit names on multiple file fields

If a file field tag is passed the multiple option, it is turned into an
array field (appending "[]"), but if the file field is passed an
explicit name as an option, leave the name alone (do not append "[]").

Fixes #9830
Merge pull request #10105 from rmm5t/fix-explicit-name-on-multiple-fields

Fix explicit names on multiple file fields