Use txt instead of txt as it directly maps to plantuml-server URLs

plantuml-mode.el

@@ -274,10 +274,10 @@
 
 (defvar plantuml-output-type
   (if (not (display-images-p))
-      "utxt"
+      "txt"
     (cond ((image-type-available-p 'svg) "svg")
           ((image-type-available-p 'png) "png")
-          (t "utxt")))
+          (t "txt")))
   "Specify the desired output type to use for generated diagrams.")
 
 (defun plantuml-read-output-type ()
@@ -287,7 +287,7 @@
           (append
            (and (image-type-available-p 'svg) '("svg"))
            (and (image-type-available-p 'png) '("png"))
-           '("utxt"))))
+           '("txt"))))
     (completing-read (format "Output type [%s]: " plantuml-output-type)
                      available-types
                      nil
@@ -306,7 +306,7 @@ default output type for new buffers."
 
 (defun plantuml-is-image-output-p ()
   "Return non-nil if the diagram output format is an image, false if it's text based."
-  (not (equal "utxt" plantuml-output-type)))
+  (not (equal "txt" plantuml-output-type)))
 
 (defun plantuml-output-type-opt ()
   "Create the flag to pass to PlantUML to produce the selected output format."

test/plantuml-preview-test.el

@@ -10,14 +10,14 @@
 
 (defun assert-preview (puml output &optional format)
   (if format
-    (setq plantuml-output-type format)
-    (setq plantuml-output-type "utxt"))
+      (setq plantuml-output-type format)
+    (setq plantuml-output-type "txt"))
   (plantuml-preview-string 42 (read-test-file puml))
   (sleep-for 3)
   (should (equal (format-preview-output (replace-regexp-in-string " " "~" (read-test-file output)))
                  (format-preview-output (replace-regexp-in-string " " "~" (read-preview-buffer))))))
 
-(ert-deftest preview-utxt-test ()
+(ert-deftest preview-txt-test ()
   (setq-local plantuml-jar-path plantuml-test-jar-path)
   (assert-preview "a-b.puml" "a-b.txt"))