diff --git a/plugins/storage/server/api/files.test.ts b/plugins/storage/server/api/files.test.ts
index dbac6720e0..3a489486ec 100644
--- a/plugins/storage/server/api/files.test.ts
+++ b/plugins/storage/server/api/files.test.ts
@@ -98,6 +98,70 @@ describe("#files.create", () => {
     expect(res.status).toEqual(400);
   });
 
+  it("should fail with status 400 if uploaded file exceeds declared size", async () => {
+    const user = await buildUser();
+    const fileName = "images.docx";
+    const attachment = await buildAttachment(
+      {
+        teamId: user.teamId,
+        userId: user.id,
+        size: 100,
+        contentType:
+          "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+      },
+      fileName
+    );
+
+    const content = await readFile(
+      path.resolve(__dirname, "..", "test", "fixtures", fileName)
+    );
+    const form = new FormData();
+    form.append("key", attachment.key);
+    form.append("file", content, fileName);
+    form.append("token", user.getJwtToken());
+
+    const res = await server.post(`/api/files.create`, {
+      headers: form.getHeaders(),
+      body: form,
+    });
+    expect(res.status).toEqual(400);
+    expect(
+      existsSync(path.join(env.FILE_STORAGE_LOCAL_ROOT_DIR, attachment.key))
+    ).toBe(false);
+  });
+
+  it("should update attachment size to actual uploaded bytes", async () => {
+    const user = await buildUser();
+    const fileName = "images.docx";
+    const attachment = await buildAttachment(
+      {
+        teamId: user.teamId,
+        userId: user.id,
+        size: 1_000_000,
+        contentType:
+          "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+      },
+      fileName
+    );
+
+    const content = await readFile(
+      path.resolve(__dirname, "..", "test", "fixtures", fileName)
+    );
+    const form = new FormData();
+    form.append("key", attachment.key);
+    form.append("file", content, fileName);
+    form.append("token", user.getJwtToken());
+
+    const res = await server.post(`/api/files.create`, {
+      headers: form.getHeaders(),
+      body: form,
+    });
+    expect(res.status).toEqual(200);
+
+    await attachment.reload();
+    expect(Number(attachment.size)).toEqual(content.byteLength);
+  });
+
   it("should succeed with status 200 ok and create a file", async () => {
     const user = await buildUser();
     const fileName = "images.docx";
diff --git a/plugins/storage/server/api/files.ts b/plugins/storage/server/api/files.ts
index 89a470e7d0..24b899f8df 100644
--- a/plugins/storage/server/api/files.ts
+++ b/plugins/storage/server/api/files.ts
@@ -2,6 +2,7 @@ import JWT from "jsonwebtoken";
 import Router from "koa-router";
 import mime from "mime-types";
 import contentDisposition from "content-disposition";
+import { bytesToHumanReadable } from "@shared/utils/files";
 import env from "@server/env";
 import {
   AuthenticationError,
@@ -52,6 +53,16 @@ router.post(
       throw AuthorizationError("Invalid key");
     }
 
+    const declaredSize = Number(attachment.size);
+
+    if (file.size > declaredSize) {
+      throw ValidationError(
+        `The uploaded file exceeds the declared size of ${bytesToHumanReadable(
+          declaredSize
+        )}`
+      );
+    }
+
     try {
       await attachment.writeFile(file);
     } catch (err) {
@@ -63,6 +74,10 @@ router.post(
       throw err;
     }
 
+    if (declaredSize !== file.size) {
+      await attachment.update({ size: file.size }, { silent: true });
+    }
+
     ctx.body = {
       success: true,
     };
diff --git a/server/routes/api/attachments/attachments.test.ts b/server/routes/api/attachments/attachments.test.ts
index 46eef4b489..27d052d172 100644
--- a/server/routes/api/attachments/attachments.test.ts
+++ b/server/routes/api/attachments/attachments.test.ts
@@ -227,6 +227,34 @@ describe("#attachments.create", () => {
       });
       expect(res.status).toEqual(400);
     });
+
+    it("should reject negative size", async () => {
+      const user = await buildUser();
+      const res = await server.post("/api/attachments.create", {
+        body: {
+          name: "test.png",
+          contentType: "image/png",
+          size: -1,
+          preset: AttachmentPreset.Emoji,
+          token: user.getJwtToken(),
+        },
+      });
+      expect(res.status).toEqual(400);
+    });
+
+    it("should reject non-integer size", async () => {
+      const user = await buildUser();
+      const res = await server.post("/api/attachments.create", {
+        body: {
+          name: "test.png",
+          contentType: "image/png",
+          size: 1.5,
+          preset: AttachmentPreset.Emoji,
+          token: user.getJwtToken(),
+        },
+      });
+      expect(res.status).toEqual(400);
+    });
   });
 
   describe("viewer", () => {
diff --git a/server/routes/api/attachments/schema.ts b/server/routes/api/attachments/schema.ts
index 35d81abe34..5fd6679775 100644
--- a/server/routes/api/attachments/schema.ts
+++ b/server/routes/api/attachments/schema.ts
@@ -26,7 +26,7 @@ export const AttachmentsCreateSchema = BaseSchema.extend({
     documentId: z.uuid().optional(),
 
     /** File size of the Attachment */
-    size: z.number(),
+    size: z.number().int().nonnegative(),
 
     /** Content-Type of the Attachment */
     contentType: z.string().optional().prefault("application/octet-stream"),
diff --git a/server/test/factories.ts b/server/test/factories.ts
index b141406a15..0315dd111a 100644
--- a/server/test/factories.ts
+++ b/server/test/factories.ts
@@ -615,7 +615,7 @@ export async function buildAttachment(
     id,
     key: AttachmentHelper.getKey({ id, name, userId: overrides.userId }),
     contentType: "image/png",
-    size: 100,
+    size: 1_000_000,
     acl,
     name,
     createdAt: new Date("2018-01-02T00:00:00.000Z"),