Looking at mini1.1.tcl in tclhttpd (https://core.tcl.tk/tclhttpd/dir?ci=ffc189660f15a776&name=bin/mini), search the the line: 

    append data(mime,$data($key)) " " $value 

it should be 
    append data(mime,$data(key)) " " $value 

From Ralf Fassel: Looks like an obvious copy/paste error to me (data(key) is set in the first branch as a lookup-cache for the second branch).

Proposed patch:

--- tclhttpd3.5.1/bin/mini/mini1.1.tcl~
+++ tclhttpd3.5.1/bin/mini/mini1.1.tcl
@@ -104,7 +104,7 @@
 		    }
 		} elseif {[regexp {^[ 	]+(.+)} $line {} value] && \
 			[info exists data(key)]} {
-		    append data(mime,$data($key)) " " $value
+		    append data(mime,$data(key)) " " $value
 		} else {
 		    HttpdError $sock 400 $line
 		}

Diff finished.  Fri Jun  7 11:29:42 2019