refactor: summarize 3 LoC into 1 in get_mimetype()

validate_workflow.sh

@@ -164,9 +164,7 @@ error() {
 
 get_mimetype() {
     local filename=$1
-    local res
-    res=$(file --mime-type "$filename" |  sed -e "s/^.*: //")
-    echo "$res"
+    echo "$( file --mime-type "$filename" |  sed -e "s/^.*: //" )"
 }