diff --git a/src/opus.cc b/src/opus.cc
index eb62e82..1c02535 100644
--- a/src/opus.cc
+++ b/src/opus.cc
@@ -182,10 +182,30 @@ std::optional<ot::picture> ot::extract_cover(const ot::opus_tags& tags)
 	return picture(decode_base64(cover_value));
 }
 
+/**
+ * Detect the MIME type of the given data block by checking the first bytes. Only the most common
+ * image formats are currently supported. Using magic(5) would give better results but that level of
+ * exhaustiveness is probably not necessary.
+ */
+static ot::byte_string_view detect_mime_type(ot::byte_string_view data)
+{
+	static std::initializer_list<std::pair<ot::byte_string_view, ot::byte_string_view>> magic_numbers = {
+		{ "\xff\xd8\xff"_bsv, "image/jpeg"_bsv },
+		{ "\x89PNG"_bsv, "image/png"_bsv },
+		{ "GIF8"_bsv, "image/gif"_bsv },
+	};
+	for (auto [magic, mime] : magic_numbers) {
+		if (data.starts_with(magic))
+			return mime;
+	}
+	fputs("warning: Could not identify the MIME type of the picture; defaulting to application/octet-stream.\n", stderr);
+	return "application/octet-stream"_bsv;
+}
+
 std::string ot::make_cover(ot::byte_string_view picture_data)
 {
 	picture pic;
-	pic.mime_type = "application/octet-stream"_bsv;
+	pic.mime_type = detect_mime_type(picture_data);
 	pic.picture_data = picture_data;
 	return "METADATA_BLOCK_PICTURE=" + encode_base64(pic.serialize());
 }
diff --git a/t/opus.cc b/t/opus.cc
index e142e39..8188530 100644
--- a/t/opus.cc
+++ b/t/opus.cc
@@ -169,16 +169,16 @@ static void make_cover()
 {
 	ot::byte_string_view picture_block = ""_bsv
 		"\x00\x00\x00\x03" // Picture type 3.
-		"\x00\x00\x00\x18" "application/octet-stream" // MIME type.
+		"\x00\x00\x00\x09" "image/png" // MIME type.
 		"\x00\x00\x00\x00" "" // Description.
 		"\x00\x00\x00\x00" // Width.
 		"\x00\x00\x00\x00" // Height.
 		"\x00\x00\x00\x00" // Color depth.
 		"\x00\x00\x00\x00" // Palette size.
-		"\x00\x00\x00\x0C" "Picture data";
+		"\x00\x00\x00\x11" "\x89PNG Picture data";
 
 	std::string expected = "METADATA_BLOCK_PICTURE=" + ot::encode_base64(picture_block);
-	is(ot::make_cover("Picture data"_bsv), expected, "build the picture tag");
+	is(ot::make_cover("\x89PNG Picture data"_bsv), expected, "build the picture tag");
 }
 
 int main()
diff --git a/t/opustags.t b/t/opustags.t
index 4915096..a9545c3 100755
--- a/t/opustags.t
+++ b/t/opustags.t
@@ -333,7 +333,7 @@ unlink('out.opus');
 
 is_deeply(opustags(qw(-D --set-cover pixel.png gobble.opus -o out.opus), ), ['', '', 0], 'set the cover');
 is_deeply(opustags(qw(--output-cover out.png out.opus), ), [<<'END_OUT', '', 0], 'extract the cover');
-METADATA_BLOCK_PICTURE=AAAAAwAAABhhcHBsaWNhdGlvbi9vY3RldC1zdHJlYW0AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEWJUE5HDQoaCgAAAA1JSERSAAAAAQAAAAEIAgAAAJB3U94AAAAMSURBVAjXY/j//z8ABf4C/tzMWecAAAAASUVORK5CYII=
+METADATA_BLOCK_PICTURE=AAAAAwAAAAlpbWFnZS9wbmcAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEWJUE5HDQoaCgAAAA1JSERSAAAAAQAAAAEIAgAAAJB3U94AAAAMSURBVAjXY/j//z8ABf4C/tzMWecAAAAASUVORK5CYII=
 END_OUT
 is(md5('out.png'), md5('pixel.png'), 'the extracted cover is identical to the one set');
 unlink('out.opus');