diff --git a/apps/theming/lib/ImageManager.php b/apps/theming/lib/ImageManager.php
index 87123a6f7c9c0..f609e5ef9d4f2 100644
--- a/apps/theming/lib/ImageManager.php
+++ b/apps/theming/lib/ImageManager.php
@@ -240,7 +240,7 @@ public function updateImage(string $key, string $tmpFile): string {
 		$supportedFormats = $this->getSupportedUploadImageFormats($key);
 		$detectedMimeType = mime_content_type($tmpFile);
 		if (!in_array($detectedMimeType, $supportedFormats, true)) {
-			throw new \Exception('Unsupported image type');
+			throw new \Exception('Unsupported image type: ' . $detectedMimeType);
 		}
 
 		if ($key === 'background' && $this->shouldOptimizeBackgroundImage($detectedMimeType, filesize($tmpFile))) {
diff --git a/apps/theming/tests/ImageManagerTest.php b/apps/theming/tests/ImageManagerTest.php
index 22432a00103f3..e0e00615edbe4 100644
--- a/apps/theming/tests/ImageManagerTest.php
+++ b/apps/theming/tests/ImageManagerTest.php
@@ -390,4 +390,30 @@ public function testUpdateImage($key, $tmpFile, $folderExists, $shouldConvert) {
 
 		$this->imageManager->updateImage($key, $tmpFile);
 	}
+
+	public function testUnsupportedImageType(): void {
+		$this->expectException(\Exception::class);
+		$this->expectExceptionMessage('Unsupported image type: text/plain');
+
+		$file = $this->createMock(ISimpleFile::class);
+		$folder = $this->createMock(ISimpleFolder::class);
+		$oldFile = $this->createMock(ISimpleFile::class);
+
+		$folder->expects($this->any())
+			->method('getFile')
+			->willReturn($oldFile);
+
+		$this->rootFolder
+			->expects($this->any())
+			->method('getFolder')
+			->with('images')
+			->willReturn($folder);
+
+		$folder->expects($this->once())
+			->method('newFile')
+			->with('favicon')
+			->willReturn($file);
+
+		$this->imageManager->updateImage('favicon', __DIR__ . '/../../../tests/data/lorem.txt');
+	}
 }