diff --git a/src/converters/ffmpeg.ts b/src/converters/ffmpeg.ts
index b8bc796..8207c61 100644
--- a/src/converters/ffmpeg.ts
+++ b/src/converters/ffmpeg.ts
@@ -1,4 +1,5 @@
-import { execFile } from "node:child_process";
+import { execFile as execFileOriginal } from "node:child_process";
+import { ExecFileFn } from "./types.ts";
 
 // This could be done dynamically by running `ffmpeg -formats` and parsing the output
 export const properties = {
@@ -691,8 +692,8 @@ export async function convert(
   fileType: string,
   convertTo: string,
   targetPath: string,
-  // eslint-disable-next-line @typescript-eslint/no-unused-vars
   options?: unknown,
+  execFile: ExecFileFn = execFileOriginal, // to make it mockable
 ): Promise<string> {
   let extraArgs: string[] = [];
   let message = "Done";
diff --git a/tests/converters/ffmpeg.test.ts b/tests/converters/ffmpeg.test.ts
new file mode 100644
index 0000000..2bb375a
--- /dev/null
+++ b/tests/converters/ffmpeg.test.ts
@@ -0,0 +1,157 @@
+import { beforeEach, expect, test } from "bun:test";
+import { convert } from "../../src/converters/ffmpeg.ts";
+
+let calls: string[][] = [];
+
+function mockExecFile(
+  _cmd: string,
+  args: string[],
+  callback: (err: Error | null, stdout: string, stderr: string) => void,
+) {
+  calls.push(args);
+  if (args.includes("fail.mov")) {
+    callback(new Error("mock failure"), "", "Fake stderr: fail");
+  } else {
+    callback(null, "Fake stdout", "");
+  }
+}
+
+beforeEach(() => {
+  calls = [];
+  delete process.env.FFMPEG_ARGS;
+});
+
+test("converts a normal file", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  const result = await convert("in.mp4", "mp4", "avi", "out.avi", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(result).toBe("Done");
+  expect(calls[0]).toEqual(expect.arrayContaining(["-i", "in.mp4", "out.avi"]));
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("adds resize for ico output", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  const result = await convert("in.png", "png", "ico", "out.ico", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(result).toBe("Done: resized to 256x256");
+  expect(calls[0]).toEqual(
+    expect.arrayContaining(["-filter:v", expect.stringContaining("scale=")]),
+  );
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("uses libaom-av1 for av1.mp4", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  await convert("in.mkv", "mkv", "av1.mp4", "out.mp4", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(calls[0]).toEqual(expect.arrayContaining(["-c:v", "libaom-av1"]));
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("uses libx264 for h264.mp4", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  await convert("in.mkv", "mkv", "h264.mp4", "out.mp4", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(calls[0]).toEqual(expect.arrayContaining(["-c:v", "libx264"]));
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("uses libx265 for h265.mp4", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  await convert("in.mkv", "mkv", "h265.mp4", "out.mp4", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(calls[0]).toEqual(expect.arrayContaining(["-c:v", "libx265"]));
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("uses libx266 for h266.mp4", async () => {
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  await convert("in.mkv", "mkv", "h266.mp4", "out.mp4", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(calls[0]).toEqual(expect.arrayContaining(["-c:v", "libx266"]));
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("respects FFMPEG_ARGS", async () => {
+  process.env.FFMPEG_ARGS = "-hide_banner -y";
+
+  const originalConsoleLog = console.log;
+
+  let loggedMessage = "";
+  console.log = (msg) => {
+    loggedMessage = msg;
+  };
+
+  await convert("input.mov", "mov", "mp4", "output.mp4", undefined, mockExecFile);
+
+  console.log = originalConsoleLog;
+
+  expect(calls[0]?.slice(0, 2)).toEqual(["-hide_banner", "-y"]);
+  expect(loggedMessage).toBe("stdout: Fake stdout");
+});
+
+test("fails on exec error", async () => {
+  const originalConsoleError = console.error;
+
+  let loggedMessage = "";
+  console.error = (msg) => {
+    loggedMessage = msg;
+  };
+
+  expect(convert("fail.mov", "mov", "mp4", "output.mp4", undefined, mockExecFile)).rejects.toThrow(
+    "mock failure",
+  );
+
+  console.error = originalConsoleError;
+
+  expect(loggedMessage).toBe("stderr: Fake stderr: fail");
+});