diff --git a/src/lib/server/services/file.ts b/src/lib/server/services/file.ts
index 8a05eb6..b3086f9 100644
--- a/src/lib/server/services/file.ts
+++ b/src/lib/server/services/file.ts
@@ -115,10 +115,8 @@ export const uploadFile = async (
   } catch (e) {
     await safeUnlink(path);
 
-    if (e instanceof IntegrityError) {
-      if (e.message === "Inactive MEK version") {
-        error(400, "Invalid MEK version");
-      }
+    if (e instanceof IntegrityError && e.message === "Inactive MEK version") {
+      error(400, "Invalid MEK version");
     }
     throw e;
   }
diff --git a/src/routes/(main)/directory/[[id]]/+page.svelte b/src/routes/(main)/directory/[[id]]/+page.svelte
index 866adbe..c5e9f8b 100644
--- a/src/routes/(main)/directory/[[id]]/+page.svelte
+++ b/src/routes/(main)/directory/[[id]]/+page.svelte
@@ -66,9 +66,9 @@
         info = getDirectoryInfo(data.id, $masterKeyStore?.get(1)?.key!);
         window.alert("파일이 업로드되었어요.");
       })
-      .catch(() => {
+      .catch((e: Error) => {
         // TODO: FIXME
-        window.alert("파일 업로드에 실패했어요.");
+        window.alert(`파일 업로드에 실패했어요.\n${e.message}`);
       });
   };
 
diff --git a/src/routes/api/file/upload/+server.ts b/src/routes/api/file/upload/+server.ts
index 66624f6..b7b38ea 100644
--- a/src/routes/api/file/upload/+server.ts
+++ b/src/routes/api/file/upload/+server.ts
@@ -6,53 +6,10 @@ import { fileUploadRequest } from "$lib/server/schemas";
 import { uploadFile } from "$lib/server/services/file";
 import type { RequestHandler } from "./$types";
 
-const parseFormData = async (contentType: string, body: ReadableStream<Uint8Array>) => {
-  return new Promise<{ metadata: string; content: Readable }>((resolve, reject) => {
-    let metadata: string | null = null;
-    let content: Readable | null = null;
+type FileMetadata = Parameters<typeof uploadFile>[0];
 
-    const bb = Busboy({ headers: { "content-type": contentType } });
-    bb.on("field", (fieldname, val) => {
-      if (fieldname !== "metadata") return reject(new Error("Invalid request body"));
-      if (metadata || content) return reject(new Error("Invalid request body")); // metadata must be first
-      metadata = val;
-    });
-    bb.on("file", (fieldname, file) => {
-      if (fieldname !== "content") return reject(new Error("Invalid request body"));
-      if (!metadata || content) return reject(new Error("Invalid request body")); // metadata must be first
-      content = file;
-      resolve({ metadata, content });
-    });
-    bb.on("finish", () => reject(new Error("Invalid request body")));
-    bb.on("error", (e) => reject(e));
-
-    body.pipeTo(Writable.toWeb(bb));
-  });
-};
-
-export const POST: RequestHandler = async ({ locals, request }) => {
-  const { userId } = await authorize(locals, "activeClient");
-
-  const contentTypeHeader = request.headers.get("Content-Type");
-  if (!contentTypeHeader?.startsWith("multipart/form-data") || !request.body) {
-    error(400, "Invalid request body");
-  }
-
-  let metadata;
-  let content;
-
-  try {
-    const formData = await parseFormData(contentTypeHeader, request.body);
-    metadata = formData.metadata;
-    content = formData.content;
-  } catch (e) {
-    if (e instanceof Error && e.message === "Invalid request body") {
-      error(400, "Invalid request body");
-    }
-    throw e;
-  }
-
-  const zodRes = fileUploadRequest.safeParse(JSON.parse(metadata));
+const parseFileMetadata = (userId: number, json: string) => {
+  const zodRes = fileUploadRequest.safeParse(JSON.parse(json));
   if (!zodRes.success) error(400, "Invalid request body");
   const {
     parentId,
@@ -73,25 +30,65 @@ export const POST: RequestHandler = async ({ locals, request }) => {
   if ((createdAt && !createdAtIv) || (!createdAt && createdAtIv))
     error(400, "Invalid request body");
 
-  await uploadFile(
-    {
-      userId,
-      parentId,
-      mekVersion,
-      encDek: dek,
-      dekVersion: new Date(dekVersion),
-      hskVersion,
-      contentHmac,
-      contentType,
-      encContentIv: contentIv,
-      encName: name,
-      encNameIv: nameIv,
-      encCreatedAt: createdAt ?? null,
-      encCreatedAtIv: createdAtIv ?? null,
-      encLastModifiedAt: lastModifiedAt,
-      encLastModifiedAtIv: lastModifiedAtIv,
-    },
-    content,
-  );
-  return text("File uploaded", { headers: { "Content-Type": "text/plain" } });
+  return {
+    userId,
+    parentId,
+    mekVersion,
+    encDek: dek,
+    dekVersion: new Date(dekVersion),
+    hskVersion,
+    contentHmac,
+    contentType,
+    encContentIv: contentIv,
+    encName: name,
+    encNameIv: nameIv,
+    encCreatedAt: createdAt ?? null,
+    encCreatedAtIv: createdAtIv ?? null,
+    encLastModifiedAt: lastModifiedAt,
+    encLastModifiedAtIv: lastModifiedAtIv,
+  } satisfies FileMetadata;
+};
+
+export const POST: RequestHandler = async ({ locals, request }) => {
+  const { userId } = await authorize(locals, "activeClient");
+
+  const contentType = request.headers.get("Content-Type");
+  if (!contentType?.startsWith("multipart/form-data") || !request.body) {
+    error(400, "Invalid request body");
+  }
+
+  return new Promise<Response>((resolve, reject) => {
+    const bb = Busboy({ headers: { "content-type": contentType } });
+    const handler =
+      <T extends unknown[]>(f: (...args: T) => Promise<void>) =>
+      (...args: T) => {
+        f(...args).catch(reject);
+      };
+
+    let metadata: FileMetadata | null = null;
+    let content: Readable | null = null;
+
+    bb.on(
+      "field",
+      handler(async (fieldname, val) => {
+        if (fieldname !== "metadata") error(400, "Invalid request body");
+        if (metadata || content) error(400, "Invalid request body");
+        metadata = parseFileMetadata(userId, val);
+      }),
+    );
+    bb.on(
+      "file",
+      handler(async (fieldname, file) => {
+        if (fieldname !== "content") error(400, "Invalid request body");
+        if (!metadata || content) error(400, "Invalid request body");
+        content = file;
+
+        await uploadFile(metadata, content);
+        resolve(text("File uploaded", { headers: { "Content-Type": "text/plain" } }));
+      }),
+    );
+    bb.on("error", (e) => content?.emit("error", e) ?? reject(e));
+
+    request.body!.pipeTo(Writable.toWeb(bb)).catch(() => {}); // busboy will handle the error
+  });
 };