diff --git a/crates/ruma-common/src/events/room.rs b/crates/ruma-common/src/events/room.rs
index f4ed1494..b3b72a0a 100644
--- a/crates/ruma-common/src/events/room.rs
+++ b/crates/ruma-common/src/events/room.rs
@@ -5,7 +5,7 @@
 use std::collections::BTreeMap;
 
 use js_int::UInt;
-use serde::{Deserialize, Serialize};
+use serde::{de, Deserialize, Serialize};
 
 use crate::{
     serde::{base64::UrlSafe, Base64},
@@ -34,7 +34,7 @@ pub mod tombstone;
 pub mod topic;
 
 /// The source of a media file.
-#[derive(Clone, Debug, Deserialize, Serialize)]
+#[derive(Clone, Debug, Serialize)]
 #[allow(clippy::exhaustive_enums)]
 pub enum MediaSource {
     /// The MXC URI to the unencrypted media file.
@@ -46,6 +46,30 @@ pub enum MediaSource {
     Encrypted(Box<EncryptedFile>),
 }
 
+/// Custom implementation of `Deserialize`, because serde doesn't guarantee what variant will be
+/// deserialized for "externally tagged"¹ enums where multiple "tag" fields exist.
+///
+/// ¹ https://serde.rs/enum-representations.html
+impl<'de> Deserialize<'de> for MediaSource {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        #[derive(Deserialize)]
+        pub struct MediaSourceJsonRepr {
+            url: Option<Box<MxcUri>>,
+            file: Option<Box<EncryptedFile>>,
+        }
+
+        match MediaSourceJsonRepr::deserialize(deserializer)? {
+            MediaSourceJsonRepr { url: None, file: None } => Err(de::Error::missing_field("url")),
+            // Prefer file if it is set
+            MediaSourceJsonRepr { file: Some(file), .. } => Ok(MediaSource::Encrypted(file)),
+            MediaSourceJsonRepr { url: Some(url), .. } => Ok(MediaSource::Plain(url)),
+        }
+    }
+}
+
 /// Metadata about an image.
 #[derive(Clone, Debug, Default, Deserialize, Serialize)]
 #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
diff --git a/crates/ruma-common/src/events/room/thumbnail_source_serde.rs b/crates/ruma-common/src/events/room/thumbnail_source_serde.rs
index d57e698c..94ca45ad 100644
--- a/crates/ruma-common/src/events/room/thumbnail_source_serde.rs
+++ b/crates/ruma-common/src/events/room/thumbnail_source_serde.rs
@@ -1,9 +1,8 @@
 //! De-/serialization functions for `Option<MediaSource>` objects representing a thumbnail source.
 
 use serde::{
-    de::Deserializer,
     ser::{SerializeStruct, Serializer},
-    Deserialize,
+    Deserialize, Deserializer,
 };
 
 use crate::MxcUri;
@@ -32,25 +31,20 @@ pub fn deserialize<'de, D>(deserializer: D) -> Result<Option<MediaSource>, D::Er
 where
     D: Deserializer<'de>,
 {
-    Option::<ThumbnailSource>::deserialize(deserializer).map(|source| source.map(Into::into))
-}
+    #[derive(Deserialize)]
+    pub struct ThumbnailSourceJsonRepr {
+        thumbnail_url: Option<Box<MxcUri>>,
+        thumbnail_file: Option<Box<EncryptedFile>>,
+    }
 
-#[derive(Clone, Debug, Deserialize)]
-enum ThumbnailSource {
-    /// The MXC URI to the unencrypted media file.
-    #[serde(rename = "thumbnail_url")]
-    Plain(Box<MxcUri>),
-
-    /// The encryption info of the encrypted media file.
-    #[serde(rename = "thumbnail_file")]
-    Encrypted(Box<EncryptedFile>),
-}
-
-impl From<ThumbnailSource> for MediaSource {
-    fn from(source: ThumbnailSource) -> Self {
-        match source {
-            ThumbnailSource::Plain(url) => Self::Plain(url),
-            ThumbnailSource::Encrypted(file) => Self::Encrypted(file),
+    match ThumbnailSourceJsonRepr::deserialize(deserializer)? {
+        ThumbnailSourceJsonRepr { thumbnail_url: None, thumbnail_file: None } => Ok(None),
+        // Prefer file if it is set
+        ThumbnailSourceJsonRepr { thumbnail_file: Some(file), .. } => {
+            Ok(Some(MediaSource::Encrypted(file)))
+        }
+        ThumbnailSourceJsonRepr { thumbnail_url: Some(url), .. } => {
+            Ok(Some(MediaSource::Plain(url)))
         }
     }
 }