diff --git a/.config/ci.yml b/.config/ci.yml
index d20ede8d35..8730ccab3a 100644
--- a/.config/ci.yml
+++ b/.config/ci.yml
@@ -229,3 +229,8 @@ checkActivityPubGetSignature: false
 
 # Upload or download file size limits (bytes)
 #maxFileSize: 262144000
+
+# CHMod-style permission bits to apply to uploaded files.
+# Permission bits are specified as a base-8 string representing User/Group/Other permissions.
+# This setting is only useful for custom deployments, such as using a reverse proxy to serve media.
+#filePermissionBits: '644'
diff --git a/.config/cypress-devcontainer.yml b/.config/cypress-devcontainer.yml
index d8013a1c95..342b0f43da 100644
--- a/.config/cypress-devcontainer.yml
+++ b/.config/cypress-devcontainer.yml
@@ -222,3 +222,8 @@ allowedPrivateNetworks: [
 
 # Upload or download file size limits (bytes)
 #maxFileSize: 262144000
+
+# CHMod-style permission bits to apply to uploaded files.
+# Permission bits are specified as a base-8 string representing User/Group/Other permissions.
+# This setting is only useful for custom deployments, such as using a reverse proxy to serve media.
+#filePermissionBits: '644'
diff --git a/.config/docker_example.yml b/.config/docker_example.yml
index 5fac3dc41e..ce2daf3aec 100644
--- a/.config/docker_example.yml
+++ b/.config/docker_example.yml
@@ -312,3 +312,8 @@ checkActivityPubGetSignature: false
 
 # Upload or download file size limits (bytes)
 #maxFileSize: 262144000
+
+# CHMod-style permission bits to apply to uploaded files.
+# Permission bits are specified as a base-8 string representing User/Group/Other permissions.
+# This setting is only useful for custom deployments, such as using a reverse proxy to serve media.
+#filePermissionBits: '644'
diff --git a/.config/example.yml b/.config/example.yml
index cf7b972de5..9debb3bf70 100644
--- a/.config/example.yml
+++ b/.config/example.yml
@@ -334,3 +334,8 @@ checkActivityPubGetSignature: false
 
 # PID File of master process
 #pidFile: /tmp/misskey.pid
+
+# CHMod-style permission bits to apply to uploaded files.
+# Permission bits are specified as a base-8 string representing User/Group/Other permissions.
+# This setting is only useful for custom deployments, such as using a reverse proxy to serve media.
+#filePermissionBits: '644'
diff --git a/packages/backend/src/config.ts b/packages/backend/src/config.ts
index 3dc49c7eb6..9cac058d96 100644
--- a/packages/backend/src/config.ts
+++ b/packages/backend/src/config.ts
@@ -115,6 +115,7 @@ type Source = {
 	};
 
 	pidFile: string;
+	filePermissionBits?: string;
 };
 
 export type Config = {
@@ -212,6 +213,7 @@ export type Config = {
 	} | undefined;
 
 	pidFile: string;
+	filePermissionBits?: string;
 };
 
 const _filename = fileURLToPath(import.meta.url);
@@ -347,6 +349,7 @@ export function loadConfig(): Config {
 		deactivateAntennaThreshold: config.deactivateAntennaThreshold ?? (1000 * 60 * 60 * 24 * 7),
 		import: config.import,
 		pidFile: config.pidFile,
+		filePermissionBits: config.filePermissionBits,
 	};
 }
 
@@ -452,7 +455,10 @@ function applyEnvOverrides(config: Source) {
 		}
 	}
 
-	const alwaysStrings = { 'chmodSocket': true } as { [key: string]: boolean };
+	const alwaysStrings: { [key in string]?: boolean } = {
+		'chmodSocket': true,
+		'filePermissionBits': true,
+	};
 
 	function _assign(path: (string | number)[], lastStep: string | number, value: string) {
 		let thisConfig = config as any;
@@ -490,7 +496,7 @@ function applyEnvOverrides(config: Source) {
 	_apply_top(['sentryForBackend', 'enableNodeProfiling']);
 	_apply_top([['clusterLimit', 'deliverJobConcurrency', 'inboxJobConcurrency', 'relashionshipJobConcurrency', 'deliverJobPerSec', 'inboxJobPerSec', 'relashionshipJobPerSec', 'deliverJobMaxAttempts', 'inboxJobMaxAttempts']]);
 	_apply_top([['outgoingAddress', 'outgoingAddressFamily', 'proxy', 'proxySmtp', 'mediaProxy', 'proxyRemoteFiles', 'videoThumbnailGenerator']]);
-	_apply_top([['maxFileSize', 'maxNoteLength', 'maxRemoteNoteLength', 'maxAltTextLength', 'maxRemoteAltTextLength', 'pidFile']]);
+	_apply_top([['maxFileSize', 'maxNoteLength', 'maxRemoteNoteLength', 'maxAltTextLength', 'maxRemoteAltTextLength', 'pidFile', 'filePermissionBits']]);
 	_apply_top(['import', ['downloadTimeout', 'maxFileSize']]);
 	_apply_top([['signToActivityPubGet', 'checkActivityPubGetSignature']]);
 }
diff --git a/packages/backend/src/core/InternalStorageService.ts b/packages/backend/src/core/InternalStorageService.ts
index 5b179baf41..b00c5796d2 100644
--- a/packages/backend/src/core/InternalStorageService.ts
+++ b/packages/backend/src/core/InternalStorageService.ts
@@ -4,7 +4,7 @@
  */
 
 import * as fs from 'node:fs';
-import { copyFile, unlink, writeFile } from 'node:fs/promises';
+import { copyFile, unlink, writeFile, chmod } from 'node:fs/promises';
 import * as Path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { dirname } from 'node:path';
@@ -41,12 +41,20 @@ export class InternalStorageService {
 	@bindThis
 	public async saveFromPath(key: string, srcPath: string): Promise<string> {
 		await copyFile(srcPath, this.resolvePath(key));
-		return `${this.config.url}/files/${key}`;
+		return await this.finalizeSavedFile(key);
 	}
 
 	@bindThis
 	public async saveFromBuffer(key: string, data: Buffer): Promise<string> {
 		await writeFile(this.resolvePath(key), data);
+		return await this.finalizeSavedFile(key);
+	}
+
+	private async finalizeSavedFile(key: string): Promise<string> {
+		if (this.config.filePermissionBits) {
+			const path = this.resolvePath(key);
+			await chmod(path, this.config.filePermissionBits);
+		}
 		return `${this.config.url}/files/${key}`;
 	}