fix: keep attachment file extension when saving

legacy/core/src/main/java/com/fsck/k9/helper/MimeTypeUtil.java

@@ -898,6 +898,26 @@ public static String getMimeTypeByExtension(String filename) {
         return DEFAULT_ATTACHMENT_MIME_TYPE;
     }
 
+    /**
+     * Returns a MIME type that is consistent with the extension of {@code displayName}.
+     *
+     * <p>This is used when creating a document via the Storage Access Framework. If the declared MIME type does not
+     * round-trip to the display name's extension, the provider appends the declared type's preferred extension (e.g.
+     * saving {@code appointment.ics} declared as {@code text/plain} would produce {@code appointment.ics.txt}). By
+     * passing a MIME type derived from the extension, the provider keeps the filename unchanged.
+     *
+     * <p>When the display name has no extension, there is nothing to preserve, so the declared MIME type (or
+     * {@link #DEFAULT_ATTACHMENT_MIME_TYPE} when it is {@code null}) is returned and the provider may add a suitable
+     * extension.
+     */
+    public static String getMimeTypeForFilename(String displayName, String declaredMimeType) {
+        if (displayName != null && displayName.lastIndexOf('.') != -1) {
+            return getMimeTypeByExtension(displayName);
+        }
+
+        return declaredMimeType != null ? declaredMimeType : DEFAULT_ATTACHMENT_MIME_TYPE;
+    }
+
     public static String getExtensionByMimeType(@NotNull String mimeType) {
         String lowerCaseMimeType = mimeType.toLowerCase(Locale.US);
         for (String[] contentTypeMapEntry : MIME_TYPE_BY_EXTENSION_MAP) {

legacy/core/src/test/java/com/fsck/k9/helper/MimeTypeUtilTest.kt

@@ -0,0 +1,53 @@
+package com.fsck.k9.helper
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+
+@RunWith(RobolectricTestRunner::class)
+class MimeTypeUtilTest {
+
+    @Test
+    fun `getMimeTypeForFilename returns extension-consistent type for a known extension`() {
+        val result = MimeTypeUtil.getMimeTypeForFilename("appointment.ics", "text/plain")
+
+        assertThat(result).isEqualTo("text/calendar")
+    }
+
+    @Test
+    fun `getMimeTypeForFilename returns extension-consistent type for a pkpass file`() {
+        val result = MimeTypeUtil.getMimeTypeForFilename("boardingpass.pkpass", "application/octet-stream")
+
+        assertThat(result).isEqualTo("application/vnd.apple.pkpass")
+    }
+
+    @Test
+    fun `getMimeTypeForFilename ignores the declared type when the extension is known`() {
+        val result = MimeTypeUtil.getMimeTypeForFilename("notes.txt", "application/octet-stream")
+
+        assertThat(result).isEqualTo("text/plain")
+    }
+
+    @Test
+    fun `getMimeTypeForFilename falls back to declared type when there is no extension`() {
+        val result = MimeTypeUtil.getMimeTypeForFilename("noname", "text/plain")
+
+        assertThat(result).isEqualTo("text/plain")
+    }
+
+    @Test
+    fun `getMimeTypeForFilename falls back to default type when there is no extension and no declared type`() {
+        val result = MimeTypeUtil.getMimeTypeForFilename("noname", null)
+
+        assertThat(result).isEqualTo("application/octet-stream")
+    }
+
+    @Test
+    fun `getMimeTypeForFilename resolves an unknown extension to the default type`() {
+        val result = MimeTypeUtil.getMimeTypeForFilename("mystery.unknownext", "text/plain")
+
+        assertThat(result).isEqualTo("application/octet-stream")
+    }
+}

legacy/ui/legacy/src/main/java/com/fsck/k9/ui/messageview/MessageViewFragment.kt

@@ -48,6 +48,7 @@ import com.fsck.k9.fragment.ConfirmationDialogFragment
 import com.fsck.k9.fragment.ConfirmationDialogFragment.ConfirmationDialogFragmentListener
 import com.fsck.k9.helper.HttpsUnsubscribeUri
 import com.fsck.k9.helper.MailtoUnsubscribeUri
+import com.fsck.k9.helper.MimeTypeUtil
 import com.fsck.k9.helper.UnsubscribeUri
 import com.fsck.k9.mail.Part
 import com.fsck.k9.mailstore.AttachmentViewInfo
@@ -1197,9 +1198,7 @@ class MessageViewFragment :
             createDocumentLauncher.launch(
                 input = CreateDocumentResultContract.Input(
                     title = attachment.displayName ?: getString(MessageReaderR.string.unnamed_attachment_title),
-                    mimeType = requireNotNull(attachment.mimeType) {
-                        "Invalid attachment type. The mimeType is null. Attachment = $attachment"
-                    },
+                    mimeType = MimeTypeUtil.getMimeTypeForFilename(attachment.displayName, attachment.mimeType),
                 ),
             )
         } catch (_: ActivityNotFoundException) {