Limit avatar upload to valid image files (#24258)

This causes the browser to allow only valid images in the file picker by
default.

---------

Co-authored-by: silverwind <me@silverwind.io>

templates/org/settings/options.tmpl

@@ -91,7 +91,7 @@
 						{{.CsrfTokenHtml}}
 						<div class="inline field">
 							<label for="avatar">{{.locale.Tr "settings.choose_new_avatar"}}</label>
-							<input name="avatar" type="file" >
+							<input name="avatar" type="file" accept="image/png,image/jpeg,image/gif,image/webp">
 						</div>
 
 						<div class="field">

templates/repo/settings/options.tmpl

@@ -60,7 +60,7 @@
 				{{.CsrfTokenHtml}}
 				<div class="inline field">
 					<label for="avatar">{{.locale.Tr "settings.choose_new_avatar"}}</label>
-					<input name="avatar" type="file" >
+					<input name="avatar" type="file" accept="image/png,image/jpeg,image/gif,image/webp">
 				</div>
 
 				<div class="field">

templates/user/settings/profile.tmpl

@@ -123,7 +123,7 @@
 
 				<div class="inline field">
 					<label for="avatar">{{.locale.Tr "settings.choose_new_avatar"}}</label>
-					<input name="avatar" type="file" >
+					<input name="avatar" type="file" accept="image/png,image/jpeg,image/gif,image/webp">
 				</div>
 
 				<div class="field">