Correctly limit theme watermark filetypes

See also relevant backend changes:
https://github.com/opencast/opencast/pull/6921/changes

This patch corrects the translation string that tells the user
what acceptable files types are. It also sets those file types
as acceptable for the file upload.

Author: Arnei

src/components/configuration/partials/wizard/WatermarkPage.tsx

@@ -75,7 +75,7 @@ const WatermarkPage = <T extends RequiredFormProps>({
 							</header>
 							<div className="obj-container padded">
 								<FileUpload
-									acceptableTypes="image/*"
+									acceptableTypes="image/gif,image/png,image/jpg,image/jpeg,image/bmp"
 									fileId="watermarkFile"
 									fileName="watermarkFileName"
 									formik={formik}

src/i18n/org/opencastproject/adminui/languages/lang-en_US.json

@@ -1672,7 +1672,7 @@
 					"BOTTOM_RIGHT": "Bottom right",
 					"UPLOAD": "Upload",
 					"POSITION": "Watermark position and preview",
-					"FILEUPLOAD_DESCRIPTION": "File should have an alpha channel (transparent background). Acceptable file types are: .PNG, .GIF, .SVG, .WEBP",
+					"FILEUPLOAD_DESCRIPTION": "File should have an alpha channel (transparent background). Acceptable file types are: gif, png, jp(e)g, or bmp",
 					"UPLOAD_LABEL": "Upload image",
 					"UPLOAD_BUTTON": "Upload"
 				},