OO-725 add missing error message when wrong poster image type is used

src/main/java/org/olat/modules/video/ui/VideoPosterUploadForm.java

@@ -73,10 +73,11 @@ public class VideoPosterUploadForm extends FormBasicController {
 		metaDataFolder = VFSManager.getOrCreateContainer(videoResourceFileroot, "media");
 
 		posterField = uifactory.addFileElement(getWindowControl(), "poster", "video.config.poster", formLayout);
-		posterField.limitToMimeType(imageMimeTypes, null, null);
+		posterField.limitToMimeType(imageMimeTypes, "poster.error.filetype", null);
 		posterField.setMaxUploadSizeKB(picUploadlimitKB, null, null);
 		posterField.setPreview(ureq.getUserSession(), true);
 		posterField.addActionListener(FormEvent.ONCHANGE);
+		posterField.setHelpTextKey("poster.help", null);
 
 		FormLayoutContainer buttonGroupLayout = FormLayoutContainer.createButtonLayout("buttonGroupLayout", getTranslator());
 		formLayout.add(buttonGroupLayout);

src/main/java/org/olat/modules/video/ui/_i18n/LocalStrings_de.properties

@@ -7,6 +7,8 @@ admin.config.videoNode=Video Kursbaustein aktivieren
 admin.menu.title=Video
 admin.menu.title.alt=Konfiguration der Video-Resource
 poster.select=Poster ausw\u00E4hlen
+poster.help=Bild vom Typ JPG. Das Bild sollte die identischen Masse wie das Original Video haben (Selbe H\u00F6he und Weite in Pixel).
+poster.error.filetype=F\u00FCr Poster Bild werden nur Bilder vom Typ JPG unterst\u00FCtzt.
 quality.table.header.dimension=Dimension
 quality.table.header.format=Format
 quality.table.header.size=Gr\u00F6sse

src/main/java/org/olat/modules/video/ui/_i18n/LocalStrings_en.properties

@@ -6,6 +6,8 @@ admin.config.transcoding=Enable video transcoding
 admin.config.videoNode=Enable video course element
 admin.menu.title=Video
 admin.menu.title.alt=Configuration of the Video-Resource
+poster.help=Image of type JPG. Make sure the poster has the exact same dimension as the original video (same height and width in pixel).
+poster.error.filetype=For poster images only images of type JPG are supported.
 poster.select=Select poster frame
 tab.video.metaDataConfig=Metadata
 tab.video.posterConfig=Replace poster