Better "upload error" message with supported extensions

src/octoprint/static/js/app/viewmodels/files.js

@@ -544,7 +544,14 @@ $(function() {
         };
 
         self._handleUploadFail = function(e, data) {
-            var error = "<p>" + gettext("Could not upload the file. Make sure that it is a GCODE file and has the extension \".gcode\" or \".gco\" or that it is an STL file with the extension \".stl\".") + "</p>";
+            var extensions = _.map(SUPPORTED_EXTENSIONS, function(extension) {
+                return extension.toLowerCase();
+            }).sort();
+            extensions = extensions.join(", ");
+            var error = "<p>"
+                + _.sprintf(gettext("Could not upload the file. Make sure that it is a valid file with one of these extensions: %(extensions)s"),
+                            {extensions: extensions})
+                + "</p>";
             error += pnotifyAdditionalInfo("<pre>" + data.jqXHR.responseText + "</pre>");
             new PNotify({
                 title: "Upload failed",

src/octoprint/templates/initscript.jinja2

@@ -29,6 +29,7 @@
     var BRANCH = "{{ branch|e }}";
     var LOCALE = "{{ g.locale|e }}";
     var AVAILABLE_LOCALES = {{ locales|tojson }};
+    var SUPPORTED_EXTENSIONS = {{ supportedExtensions|tojson }};
 
     var OCTOPRINT_VIEWMODELS = [];
     var ADDITIONAL_VIEWMODELS = [];