diff --git a/lib/private/StreamImage.php b/lib/private/StreamImage.php
index 233a48e8158..33078310d27 100644
--- a/lib/private/StreamImage.php
+++ b/lib/private/StreamImage.php
@@ -37,7 +37,7 @@ class StreamImage implements IStreamImage {
 	/** @var resource The internal stream */
 	private $stream;
 
-	/** @var string */
+	/** @var null|string */
 	private $mimeType;
 
 	/** @var int */
diff --git a/lib/private/legacy/OC_Image.php b/lib/private/legacy/OC_Image.php
index 1e6ff72fb42..9ccc6409ba0 100644
--- a/lib/private/legacy/OC_Image.php
+++ b/lib/private/legacy/OC_Image.php
@@ -60,7 +60,7 @@ class OC_Image implements \OCP\IImage {
 	protected $resource = false; // tmp resource.
 	/** @var int */
 	protected $imageType = IMAGETYPE_PNG; // Default to png if file type isn't evident.
-	/** @var string */
+	/** @var null|string */
 	protected $mimeType = 'image/png'; // Default to png
 	/** @var null|string */
 	protected $filePath = null;
@@ -116,12 +116,12 @@ class OC_Image implements \OCP\IImage {
 	}
 
 	/**
-	 * Returns the MIME type of the image or an empty string if no image is loaded.
+	 * Returns the MIME type of the image or null if no image is loaded.
 	 *
 	 * @return string
 	 */
-	public function mimeType(): string {
-		return $this->valid() ? $this->mimeType : '';
+	public function mimeType(): ?string {
+		return $this->valid() ? $this->mimeType : null;
 	}
 
 	/**
@@ -354,12 +354,11 @@ class OC_Image implements \OCP\IImage {
 	}
 
 	/**
-	 * @return string Returns the mimetype of the data. Returns the empty string
-	 * if the data is not valid.
+	 * @return string Returns the mimetype of the data. Returns null if the data is not valid.
 	 */
-	public function dataMimeType(): string {
+	public function dataMimeType(): ?string {
 		if (!$this->valid()) {
-			return '';
+			return null;
 		}
 
 		switch ($this->mimeType) {
diff --git a/lib/public/IImage.php b/lib/public/IImage.php
index 43db00a8bd1..f1ac3bf1a50 100644
--- a/lib/public/IImage.php
+++ b/lib/public/IImage.php
@@ -41,11 +41,11 @@ interface IImage {
 	public function valid(): bool;
 
 	/**
-	 * Returns the MIME type of the image or an empty string if no image is loaded.
+	 * Returns the MIME type of the image or null if no image is loaded.
 	 *
 	 * @since 8.1.0
 	 */
-	public function mimeType(): string;
+	public function mimeType(): ?string;
 
 	/**
 	 * Returns the width of the image or -1 if no image is loaded.
@@ -98,11 +98,11 @@ interface IImage {
 	public function resource();
 
 	/**
-	 * @return string Returns the mimetype of the data. Returns the empty string
+	 * @return string Returns the mimetype of the data. Returns null
 	 * if the data is not valid.
 	 * @since 13.0.0
 	 */
-	public function dataMimeType(): string;
+	public function dataMimeType(): ?string;
 
 	/**
 	 * @return string Returns the raw image data.