templates: fix not able to upload issue attachments (#5978)

templates/repo/issue/comment_tab.tmpl

@@ -13,5 +13,5 @@
 </div>
 {{if .IsAttachmentEnabled}}
 	<div class="files"></div>
-	<div class="ui basic button dropzone" id="dropzone" data-upload-url="{{AppSubURL}}/issues/attachments" data-accepts="{{.AttachmentAllowedTypes}}" data-max-file="{{.AttachmentMaxFiles}}" data-max-size="{{.AttachmentMaxSize}}" data-default-message="{{.i18n.Tr "dropzone.default_message"}}" data-invalid-input-type="{{.i18n.Tr "dropzone.invalid_input_type"}}" data-file-too-big="{{.i18n.Tr "dropzone.file_too_big"}}" data-remove-file="{{.i18n.Tr "dropzone.remove_file"}}"></div>
+	<div class="ui basic button dropzone" id="dropzone" data-upload-url="{{AppSubURL}}/issues/attachments" data-accepts="{{Join .AttachmentAllowedTypes ","}}" data-max-file="{{.AttachmentMaxFiles}}" data-max-size="{{.AttachmentMaxSize}}" data-default-message="{{.i18n.Tr "dropzone.default_message"}}" data-invalid-input-type="{{.i18n.Tr "dropzone.invalid_input_type"}}" data-file-too-big="{{.i18n.Tr "dropzone.file_too_big"}}" data-remove-file="{{.i18n.Tr "dropzone.remove_file"}}"></div>
 {{end}}