I have a fileshare with several thousands .config files (xml files) - and several thousands of other files. I added the .config files to the allowed file-types and restarted search.
Apparently the search crawler (SP2013) parses the content and discovers that they are application/xml - thats why I can't search for tag names, eg.
<settings> because only xml values are indexed.
I tried to override the xml mimetype with
New-SPEnterpriseSearchFileFormat -SearchApplication $ssa config "Config" text/plain
But that does not change my result...