Using a specific file type with a specific file field

I've got two file types - the default Document and a second custom file type, Vacancy Application. They both are tied to PDF and Word documents. I would like to use the Document file type on one particular file field and Vacancy Application on another file field. I can't see any current way of specifying which file type those file fields use. As it stands, having just created the custom Vacancy Application file type, it seems to be overriding the Document file type on all pdf/word document file uploads.

Is there any way I can specify which file field uses which file type?

Cheers

welly