Determine supported image file types by checking the file header

Previously, the image format was determined by checking the file extension.
This commit updates the implementation to determine the image format by checking
the file header.

The format of a file is determined by its internal bit layout rather than its extension. File extensions are merely human-readable metadata and do not reflect the actual contents of a file. For instance, renaming a .jpg file to another extension does not alter its internal format and could result in a file of a completely different type, such as a harmful executable.

Relying solely on file extensions to determine file format is unreliable, as it may lead to incorrect results. Instead, file headers contain unique, format-specific signatures that provide a more accurate and stable method for identifying a file's true nature.

src/image.c

@@ -31,27 +31,55 @@ typedef enum {
 #undef _
 } twin_image_format_t;
 
-/* FIXME: Check the header of the given images to determine the supported image
- * formats instead of parsing the filename without checking its content.
+/*
+ * Defines the headers of supported image formats.
+ * Each image format has a unique header, allowing the format to be determined
+ * by inspecting the file header.
+ * Supported formats: PNG, JPEG.
+ * Reference:
+ * - PNG:
+ * http://www.libpng.org/pub/png/spec/1.2/PNG-Rationale.html#R.PNG-file-signature
+ * - JPEG:
+ * https://www.file-recovery.com/jpg-signature-format.htm
  */
-static twin_image_format_t image_type_from_name(const char *path)
+#if __BYTE_ORDER == __BIG_ENDIAN
+static const uint8_t header_png[8] = {
+    0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A,
+};
+#else
+static const uint8_t header_png[8] = {
+    0x0A, 0x1A, 0x0A, 0x0D, 0x47, 0x4E, 0x50, 0x89,
+};
+#endif
+static const uint8_t header_jpeg[3] = {0xFF, 0xD8, 0xFF};
+
+static twin_image_format_t image_type_detect(const char *path)
 {
     twin_image_format_t type = IMAGE_TYPE_unknown;
-    const char *extname = strrchr(path, '.');
-    if (!extname)
+    FILE *file = fopen(path, "rb");
+    if (!file) {
+        fprintf(stderr, "Failed to open %s\n", path);
+        return IMAGE_TYPE_unknown;
+    }
+
+    uint8_t header[8];
+    size_t bytes_read = fread(header, 1, sizeof(header), file);
+    fclose(file);
+
+    if (bytes_read < 8) /* incomplete image file */
         return IMAGE_TYPE_unknown;
 #if LOADER_HAS(PNG)
-    else if (!strcasecmp(extname, ".png")) {
+    else if (!memcmp(header, header_png, sizeof(header_png))) {
         type = IMAGE_TYPE_png;
     }
 #endif
 #if LOADER_HAS(JPEG)
-    else if (!strcasecmp(extname, ".jpg") || !strcasecmp(extname, ".jpeg")) {
+    else if (!memcmp(header, header_jpeg, sizeof(header_jpeg))) {
         type = IMAGE_TYPE_jpeg;
     }
 #endif
-    /* otherwise, unsupported format */
 
+    /* otherwise, unsupported format */
     return type;
 }
 
@@ -74,7 +102,7 @@ static loader_func_t image_loaders[] = {
 
 twin_pixmap_t *twin_pixmap_from_file(const char *path, twin_format_t fmt)
 {
-    loader_func_t loader = image_loaders[image_type_from_name(path)];
+    loader_func_t loader = image_loaders[image_type_detect(path)];
     if (!loader)
         return NULL;
     return loader(path, fmt);