test: add media extension detection tests for ShareHelper

Make getImageExtension, getVideoExtension, and getMediaExtension
internal for testing. Add tests covering all supported image formats
(JPEG, PNG, GIF, WebP) and video formats (WebM, AVI, MP4, MOV),
plus edge cases (unknown formats, empty files, short headers).

https://claude.ai/code/session_01Euj5mXfjneNCx9m49YTp92

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/components/ShareHelper.kt

@@ -85,11 +85,11 @@ object ShareHelper {
             } ?: throw IOException("Unable to open snapshot for: $imageUrl")
         }
 
-    private fun getImageExtension(file: File): String = getMediaExtension(file, isVideo = false)
+    internal fun getImageExtension(file: File): String = getMediaExtension(file, isVideo = false)
 
-    private fun getVideoExtension(file: File): String = getMediaExtension(file, isVideo = true)
+    internal fun getVideoExtension(file: File): String = getMediaExtension(file, isVideo = true)
 
-    private fun getMediaExtension(
+    internal fun getMediaExtension(
         file: File,
         isVideo: Boolean,
     ): String {

amethyst/src/test/java/com/vitorpamplona/amethyst/ui/components/ShareHelperTest.kt

@@ -49,4 +49,117 @@ class ShareHelperTest {
         targetFile.delete()
         tempDir.delete()
     }
+
+    @Test
+    fun getImageExtension_jpeg() {
+        val file = createTempFileWithBytes(byteArrayOf(0xFF.toByte(), 0xD8.toByte(), 0x00, 0x00))
+        assertEquals("jpg", ShareHelper.getImageExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getImageExtension_png() {
+        val file = createTempFileWithBytes(byteArrayOf(0x89.toByte(), 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A))
+        assertEquals("png", ShareHelper.getImageExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getImageExtension_gif() {
+        val file = createTempFileWithBytes("GIF89a".toByteArray())
+        assertEquals("gif", ShareHelper.getImageExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getImageExtension_webp() {
+        // RIFF....WEBP
+        val header = ByteArray(12)
+        "RIFF".toByteArray().copyInto(header, 0)
+        "WEBP".toByteArray().copyInto(header, 8)
+        val file = createTempFileWithBytes(header)
+        assertEquals("webp", ShareHelper.getImageExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getImageExtension_unknownDefaultsToJpg() {
+        val file = createTempFileWithBytes(byteArrayOf(0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07))
+        assertEquals("jpg", ShareHelper.getImageExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getImageExtension_tooShortDefaultsToJpg() {
+        val file = createTempFileWithBytes(byteArrayOf(0x00, 0x01))
+        assertEquals("jpg", ShareHelper.getImageExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getVideoExtension_webm() {
+        val file = createTempFileWithBytes(byteArrayOf(0x1A, 0x45, 0xDF.toByte(), 0xA3.toByte(), 0x00, 0x00))
+        assertEquals("webm", ShareHelper.getVideoExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getVideoExtension_avi() {
+        // RIFF....AVI
+        val header = ByteArray(12)
+        "RIFF".toByteArray().copyInto(header, 0)
+        "AVI ".toByteArray().copyInto(header, 8)
+        val file = createTempFileWithBytes(header)
+        assertEquals("avi", ShareHelper.getVideoExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getVideoExtension_mp4Isom() {
+        // ....ftypisom
+        val header = ByteArray(12)
+        "ftyp".toByteArray().copyInto(header, 4)
+        "isom".toByteArray().copyInto(header, 8)
+        val file = createTempFileWithBytes(header)
+        assertEquals("mp4", ShareHelper.getVideoExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getVideoExtension_mov() {
+        // ....ftypqt__
+        val header = ByteArray(12)
+        "ftyp".toByteArray().copyInto(header, 4)
+        "qt  ".toByteArray().copyInto(header, 8)
+        val file = createTempFileWithBytes(header)
+        assertEquals("mov", ShareHelper.getVideoExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getVideoExtension_unknownDefaultsToMp4() {
+        val file = createTempFileWithBytes(byteArrayOf(0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07))
+        assertEquals("mp4", ShareHelper.getVideoExtension(file))
+        file.delete()
+    }
+
+    @Test
+    fun getMediaExtension_emptyFileDefaultsToImage() {
+        val file = createTempFileWithBytes(byteArrayOf())
+        assertEquals("jpg", ShareHelper.getMediaExtension(file, isVideo = false))
+        file.delete()
+    }
+
+    @Test
+    fun getMediaExtension_emptyFileDefaultsToVideo() {
+        val file = createTempFileWithBytes(byteArrayOf())
+        assertEquals("mp4", ShareHelper.getMediaExtension(file, isVideo = true))
+        file.delete()
+    }
+
+    private fun createTempFileWithBytes(bytes: ByteArray): File {
+        val file = File.createTempFile("sharehelpertest", ".tmp")
+        file.writeBytes(bytes)
+        return file
+    }
 }