Send a concrete mime type for media shared with a wildcard one

Some apps share an image with the mime type image/* rather than a
concrete one. That value was carried straight into the uploaded event, so
the message reached the room with an unusable media type and bridges
rejected it as unsupported.

Resolve a wildcard subtype to a default for its top level type before
processing the upload, reusing the mapping the download path already
applies so the SDK can pick the right extension.

libraries/core/src/main/kotlin/io/element/android/libraries/core/mimetype/MimeTypes.kt

@@ -63,4 +63,17 @@ object MimeTypes {
         val subType = components.last()
         return subType.isNotBlank() && subType != "*"
     }
+
+    /**
+     * A mime type with a wildcard subtype is a filter, not a media type, so replace it with a
+     * concrete default for its top level type.
+     */
+    fun String?.withDefaultSubtype(): String = when {
+        this == null -> OctetStream
+        hasSubtype(this) -> this
+        this == Images -> Jpeg
+        this == Videos -> Mp4
+        this == Audio -> Mp3
+        else -> OctetStream
+    }
 }

libraries/core/src/test/kotlin/io/element/android/libraries/core/mimetype/MimeTypesTest.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.core.mimetype
+
+import com.google.common.truth.Truth.assertThat
+import io.element.android.libraries.core.mimetype.MimeTypes.withDefaultSubtype
+import org.junit.Test
+
+class MimeTypesTest {
+    @Test
+    fun `withDefaultSubtype keeps a mime type that already has a subtype`() {
+        assertThat(MimeTypes.Png.withDefaultSubtype()).isEqualTo(MimeTypes.Png)
+        assertThat(MimeTypes.Mp4.withDefaultSubtype()).isEqualTo(MimeTypes.Mp4)
+        assertThat(MimeTypes.Pdf.withDefaultSubtype()).isEqualTo(MimeTypes.Pdf)
+    }
+
+    @Test
+    fun `withDefaultSubtype replaces a wildcard subtype with a concrete one`() {
+        assertThat(MimeTypes.Images.withDefaultSubtype()).isEqualTo(MimeTypes.Jpeg)
+        assertThat(MimeTypes.Videos.withDefaultSubtype()).isEqualTo(MimeTypes.Mp4)
+        assertThat(MimeTypes.Audio.withDefaultSubtype()).isEqualTo(MimeTypes.Mp3)
+    }
+
+    @Test
+    fun `withDefaultSubtype falls back to octet stream`() {
+        assertThat(MimeTypes.Any.withDefaultSubtype()).isEqualTo(MimeTypes.OctetStream)
+        assertThat("text/*".withDefaultSubtype()).isEqualTo(MimeTypes.OctetStream)
+        assertThat(null.withDefaultSubtype()).isEqualTo(MimeTypes.OctetStream)
+    }
+}