diff --git a/lib/actions/upload-actions.js b/lib/actions/upload-actions.js
--- a/lib/actions/upload-actions.js
+++ b/lib/actions/upload-actions.js
@@ -7,7 +7,10 @@
 import { storeEstablishedHolderActionType } from './holder-actions.js';
 import blobService from '../facts/blob-service.js';
 import type { CallSingleKeyserverEndpoint } from '../keyserver-conn/call-single-keyserver-endpoint.js';
-import { extractKeyserverIDFromIDOptional } from '../keyserver-conn/keyserver-call-utils.js';
+import {
+  extractKeyserverIDFromID,
+  extractKeyserverIDFromIDOptional,
+} from '../keyserver-conn/keyserver-call-utils.js';
 import { useKeyserverCall } from '../keyserver-conn/keyserver-call.js';
 import type { CallKeyserverEndpoint } from '../keyserver-conn/keyserver-conn-types.js';
 import { type PerformHTTPMultipartUpload } from '../keyserver-conn/multipart-upload.js';
@@ -327,9 +330,122 @@
   return useKeyserverCall(blobUploadAction);
 }
 
+export type ThickThreadMediaMetadataInput = {
+  +blobHash: string,
+  +encryptionKey: string,
+  +mimeType: string,
+  +dimensions: ?Dimensions,
+  +filename?: ?string,
+  +thumbHash?: ?string,
+  +loop?: boolean,
+};
+
+export type MediaMetadataReassignmentAction = (input: {
+  +mediaMetadataInput: ThickThreadMediaMetadataInput,
+  +keyserverOrThreadID: string,
+}) => Promise<UploadMultimediaResult>;
+
+const reassignThickThreadMediaForThinThread =
+  (
+    callKeyserverEndpoint: CallKeyserverEndpoint,
+    authMetadata: AuthMetadata,
+  ): MediaMetadataReassignmentAction =>
+  async input => {
+    const { mediaMetadataInput, keyserverOrThreadID } = input;
+    const { encryptionKey, loop, dimensions, thumbHash, mimeType } =
+      mediaMetadataInput;
+    const blobHolder = generateBlobHolder();
+    const blobHash = toBase64URL(mediaMetadataInput.blobHash);
+    const defaultHeaders = createDefaultHTTPRequestHeaders(authMetadata);
+
+    let filename = mediaMetadataInput.filename;
+    if (!filename) {
+      const basename = Math.random().toString(36).slice(-10);
+      const extension = mediaConfig[mimeType]?.extension;
+      filename = extension ? `${basename}.${extension}` : basename;
+    }
+
+    // 1. Assign new holder for blob with given blobHash
+    try {
+      const assignHolderEndpoint = blobService.httpEndpoints.ASSIGN_HOLDER;
+      const assignHolderResponse = await fetch(
+        makeBlobServiceEndpointURL(assignHolderEndpoint),
+        {
+          method: assignHolderEndpoint.method,
+          body: JSON.stringify({
+            holder: blobHolder,
+            blob_hash: blobHash,
+          }),
+          headers: {
+            ...defaultHeaders,
+            'content-type': 'application/json',
+          },
+        },
+      );
+      handleHTTPResponseError(assignHolderResponse);
+    } catch (e) {
+      throw new Error(
+        `Failed to assign holder: ${
+          getMessageForException(e) ?? 'unknown error'
+        }`,
+      );
+    }
+
+    // 2. Upload media metadata to keyserver
+    const keyserverID = extractKeyserverIDFromID(keyserverOrThreadID);
+    const requests = {
+      [keyserverID]: {
+        blobHash,
+        blobHolder,
+        encryptionKey,
+        filename,
+        mimeType,
+        loop,
+        thumbHash,
+        ...dimensions,
+      },
+    };
+    const responses = await callKeyserverEndpoint(
+      'upload_media_metadata',
+      requests,
+    );
+    const response = responses[keyserverID];
+
+    return {
+      id: response.id,
+      uri: response.uri,
+      mediaType: response.mediaType,
+      dimensions: response.dimensions,
+      loop: response.loop,
+    };
+  };
+
+function useMediaMetadataReassignment(): MediaMetadataReassignmentAction {
+  const identityContext = React.useContext(IdentityClientContext);
+  invariant(identityContext, 'Identity context should be set');
+  const { getAuthMetadata } = identityContext;
+
+  const thickThreadMediaReassignmentAction = React.useCallback(
+    (
+      callSingleKeyserverEndpoint: CallSingleKeyserverEndpoint,
+    ): MediaMetadataReassignmentAction =>
+      async input => {
+        const authMetadata = await getAuthMetadata();
+        const authenticatedAction = reassignThickThreadMediaForThinThread(
+          callSingleKeyserverEndpoint,
+          authMetadata,
+        );
+        return authenticatedAction(input);
+      },
+    [getAuthMetadata],
+  );
+  return useKeyserverCall(thickThreadMediaReassignmentAction);
+}
+
 export {
   uploadMultimedia,
   useBlobServiceUpload,
+  useMediaMetadataReassignment,
   updateMultimediaMessageMediaActionType,
   useDeleteUpload,
 };