fix(client/import): file remains from previous instance (closes #7428)

apps/client/src/widgets/dialogs/import.tsx

@@ -51,7 +51,10 @@ export default function ImportDialog() {
                 setShown(false);
                 await importService.uploadFiles("notes", parentNoteId, Array.from(files), options);
             }}
-            onHidden={() => setShown(false)}
+            onHidden={() => {
+                setShown(false);
+                setFiles(null);
+            }}
             footer={<Button text={t("import.import")} primary disabled={!files} />}
             show={shown}
         >

apps/client/src/widgets/dialogs/upload_attachments.tsx

@@ -47,7 +47,10 @@ export default function UploadAttachmentsDialog() {
                 setIsUploading(false);
                 setShown(false);
             }}
-            onHidden={() => setShown(false)}
+            onHidden={() => {
+                setShown(false);
+                setFiles(null);
+            }}
             show={shown}
         >
             <FormGroup name="files" label={t("upload_attachments.choose_files")} description={description}>

apps/client/src/widgets/react/FormFileUpload.tsx

@@ -1,6 +1,6 @@
 import { Ref } from "preact";
 import Button, { ButtonProps } from "./Button";
-import { useRef } from "preact/hooks";
+import { useEffect, useRef } from "preact/hooks";
 
 interface FormFileUploadProps {
     name?: string;
@@ -11,6 +11,11 @@ interface FormFileUploadProps {
 }
 
 export default function FormFileUpload({ inputRef, name, onChange, multiple, hidden }: FormFileUploadProps) {
+    // Prevent accidental reuse of a file selected in a previous instance of the upload form.
+    useEffect(() => {
+        onChange(null);
+    }, []);
+
     return (
         <label class="tn-file-input tn-input-field" style={hidden ? { display: "none" } : undefined}>
             <input