Thread:Project:Support desk/Error: File extension does not match MIME type/reply (10)

You may also want to look at the include/mime.types file if the above fixes haven't worked.

My issue was the delta between the default apache mime.types and this file, specifically:

text/plain					txt text conf def list log in
 * 1) From Apache

text/plain txt
 * 1) From include/mime.types