gh-122 Look for, and utilize DFU "contenttype"
- Explicitly reject XML formatted files
- request dfuinfo v 1.21 (contenttype)
- If contentype available, override dfuinfo "format"
- If contentype not available, continue to use format

Signed-off-by: Rodrigo Pastrana <Rodrigo.Pastrana@lexisnexis.com>
Merge pull request #124 from rpastrana/gh-122-dfu-contenttype

gh-122 Look for, and utilize DFU "contenttype"