diff --git a/lib/media/file-utils.js b/lib/media/file-utils.js
--- a/lib/media/file-utils.js
+++ b/lib/media/file-utils.js
@@ -172,6 +172,16 @@
   return replaceExtension(filename, ext);
 }
 
+const basenameRegex = /[^a-z0-9._-]/gi;
+function sanitizeFilename(filename: ?string, mime: string): string {
+  if (!filename) {
+    // Generate a random filename and deduce the extension from the mime type.
+    const randomName = Math.random().toString(36).slice(-5);
+    filename = readableFilename(randomName, mime) || randomName;
+  }
+  return filename.replace(basenameRegex, '_');
+}
+
 const extRegex = /\.([0-9a-z]+)$/i;
 function extensionFromFilename(filename: string): ?string {
   const matches = filename.match(extRegex);
@@ -212,6 +222,7 @@
   fileInfoFromData,
   replaceExtension,
   readableFilename,
+  sanitizeFilename,
   extensionFromFilename,
   pathFromURI,
   filenameFromPathOrURI,
diff --git a/lib/types/media-types.js b/lib/types/media-types.js
--- a/lib/types/media-types.js
+++ b/lib/types/media-types.js
@@ -213,7 +213,7 @@
   | {
       +step: 'photo_library',
       +dimensions: Dimensions,
-      +filename: string,
+      +filename: ?string,
       +uri: string,
       +mediaNativeID: string,
       +selectTime: number, // ms timestamp
@@ -223,7 +223,7 @@
   | {
       +step: 'video_library',
       +dimensions: Dimensions,
-      +filename: string,
+      +filename: ?string,
       +uri: string,
       +mediaNativeID: string,
       +selectTime: number, // ms timestamp
diff --git a/native/media/media-utils.js b/native/media/media-utils.js
--- a/native/media/media-utils.js
+++ b/native/media/media-utils.js
@@ -3,7 +3,7 @@
 import invariant from 'invariant';
 import { Image } from 'react-native';
 
-import { pathFromURI, readableFilename } from 'lib/media/file-utils';
+import { pathFromURI, sanitizeFilename } from 'lib/media/file-utils';
 import type {
   Dimensions,
   MediaMissionStep,
@@ -89,8 +89,7 @@
     );
     const shouldDisposePath =
       initialURI !== uploadURI ? pathFromURI(uploadURI) : null;
-    const filename = readableFilename(selection.filename, mime);
-    invariant(filename, `could not construct filename for ${mime}`);
+    const filename = sanitizeFilename(selection.filename, mime);
     if (mediaType === 'video') {
       invariant(uploadThumbnailURI, 'video should have uploadThumbnailURI');
       sendResult({
@@ -170,11 +169,12 @@
   }
 
   if (mediaType === 'video') {
+    const filename = sanitizeFilename(selection.filename, mime);
     const { steps: videoSteps, result: videoResult } = await processVideo(
       {
         uri: initialURI,
         mime,
-        filename: selection.filename,
+        filename,
         fileSize,
         dimensions,
         hasWiFi: config.hasWiFi,