MM-22967 : Fixing file upload error feedback from server (#22972)

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/actions/file_actions.ts

@@ -87,6 +87,20 @@ export function uploadFile({file, name, type, rootId, channelId, clientId, onPro
                     ]));
 
                     onSuccess(response, channelId, rootId);
+                } else if (xhr.status >= 400 && xhr.readyState === 4) {
+                    const errorResponse = JSON.parse(xhr.response);
+                    const errorMessage =
+                        (errorResponse?.id && errorResponse?.message) ? localizeMessage(errorResponse.id, errorResponse.message) :
+                            localizeMessage('file_upload.generic_error', 'There was a problem uploading your files.');
+
+                    dispatch({
+                        type: FileTypes.UPLOAD_FILES_FAILURE,
+                        clientIds: [clientId],
+                        channelId,
+                        rootId,
+                    });
+
+                    onError?.(errorMessage, clientId, channelId, rootId);
                 }
             };
         }

webapp/channels/src/components/advanced_create_post/advanced_create_post.tsx

@@ -998,9 +998,9 @@ class AdvancedCreatePost extends React.PureComponent<Props, State> {
     };
 
     handleUploadError = (err: string | ServerError, clientId?: string, channelId?: string) => {
-        let serverError = null;
-        if (typeof err === 'string' && err.length > 0) {
-            serverError = new Error(err);
+        let serverError = err;
+        if (typeof serverError === 'string') {
+            serverError = new Error(serverError);
         }
 
         if (!channelId || !clientId) {