For tab-delimited files, http://www.rfc-editor.org/rfc/rfc4180.txt mentions ‘text/tab-separated-values’.
That format is (sparsely) described at http://www.iana.org/assignments/media-types/text/tab-separated-values.
More info can be found at : http://www.cs.tut.fi/~jkorpela/TSV.html
I hope this helps?