diff --git a/proto/backend.proto b/proto/backend.proto
index d4dfd3553..fa73d4c7f 100644
--- a/proto/backend.proto
+++ b/proto/backend.proto
@@ -97,6 +97,8 @@ message SyncError {
         CLIENT_TOO_OLD = 3;
         AUTH_FAILED = 4;
         SERVER_MESSAGE = 5;
+        MEDIA_CHECK_REQUIRED = 6;
+        RESYNC_REQUIRED = 7;
     }
     SyncErrorKind kind = 2;
 }
diff --git a/qt/aqt/mediasync.py b/qt/aqt/mediasync.py
index e4a01d9bc..0dbef445a 100644
--- a/qt/aqt/mediasync.py
+++ b/qt/aqt/mediasync.py
@@ -44,20 +44,6 @@ class MediaSyncState:
     removed_files: int = 0
 
 
-# fixme: sync.rs fixmes
-# fixme: maximum size when uploading
-# fixme: concurrent modifications during upload step
-# fixme: mediaSanity
-#         elif evt == "mediaSanity":
-#         showWarning(
-#             _(
-#                 """\
-# A problem occurred while syncing media. Please use Tools>Check Media, then \
-# sync again to correct the issue."""
-#             )
-#         )
-
-
 LogEntry = Union[MediaSyncState, str]
 
 
@@ -170,6 +156,14 @@ class MediaSyncer:
                         "AnkiWeb encountered a problem. Please try again in a few minutes."
                     )
                 )
+            elif kind == SyncErrorKind.MEDIA_CHECK_REQUIRED:
+                showWarning(_("Please use the Tools>Check Media menu option."))
+            elif kind == SyncErrorKind.RESYNC_REQUIRED:
+                showWarning(
+                    _(
+                        "Please sync again, and post on the support forum if this message keeps appearing."
+                    )
+                )
             else:
                 showWarning(_("Unexpected error: {}").format(str(exc)))
         elif isinstance(exc, NetworkError):
diff --git a/rslib/src/backend.rs b/rslib/src/backend.rs
index 377d0c4d5..e0bf8f909 100644
--- a/rslib/src/backend.rs
+++ b/rslib/src/backend.rs
@@ -87,6 +87,8 @@ impl std::convert::From<SyncErrorKind> for i32 {
             SyncErrorKind::ClientTooOld => V::ClientTooOld,
             SyncErrorKind::AuthFailed => V::AuthFailed,
             SyncErrorKind::ServerMessage => V::ServerMessage,
+            SyncErrorKind::MediaCheckRequired => V::MediaCheckRequired,
+            SyncErrorKind::ResyncRequired => V::ResyncRequired,
             SyncErrorKind::Other => V::Other,
         }) as i32
     }
diff --git a/rslib/src/err.rs b/rslib/src/err.rs
index 5df5ea266..303e24b98 100644
--- a/rslib/src/err.rs
+++ b/rslib/src/err.rs
@@ -129,6 +129,8 @@ pub enum SyncErrorKind {
     AuthFailed,
     ServerMessage,
     Other,
+    MediaCheckRequired,
+    ResyncRequired,
 }
 
 fn error_for_status_code(info: String, code: StatusCode) -> AnkiError {
diff --git a/rslib/src/media/database.rs b/rslib/src/media/database.rs
index bf160d50c..85d4dc384 100644
--- a/rslib/src/media/database.rs
+++ b/rslib/src/media/database.rs
@@ -244,6 +244,12 @@ delete from media where fname=?"
             .collect();
         Ok(map?)
     }
+
+    pub(super) fn force_resync(&mut self) -> Result<()> {
+        self.db
+            .execute_batch("delete from media; update meta set lastUsn=0, usn=0")
+            .map_err(Into::into)
+    }
 }
 
 #[cfg(test)]
diff --git a/rslib/src/media/sync.rs b/rslib/src/media/sync.rs
index dfbd467d2..2bbdb58a1 100644
--- a/rslib/src/media/sync.rs
+++ b/rslib/src/media/sync.rs
@@ -1,7 +1,7 @@
 // Copyright: Ankitects Pty Ltd and contributors
 // License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
 
-use crate::err::{AnkiError, Result};
+use crate::err::{AnkiError, Result, SyncErrorKind};
 use crate::media::database::{MediaDatabaseContext, MediaDatabaseMetadata, MediaEntry};
 use crate::media::files::{
     add_file_from_ankiweb, data_for_file, normalize_filename, remove_files, AddedFile,
@@ -22,6 +22,10 @@ use std::{io, time};
 
 static SYNC_MAX_FILES: usize = 25;
 static SYNC_MAX_BYTES: usize = (2.5 * 1024.0 * 1024.0) as usize;
+static SYNC_SINGLE_FILE_MAX_BYTES: usize = 100 * 1024 * 1024;
+
+// fixme: non-normalized filenames on ankiweb
+// fixme: concurrent modifications during upload step
 
 /// The counts are not cumulative - the progress hook should accumulate them.
 #[derive(Debug)]
@@ -182,8 +186,11 @@ where
             if data == "OK" {
                 Ok(())
             } else {
-                // fixme: force resync, handle better
-                Err(AnkiError::server_message("resync required"))
+                self.ctx.transact(|ctx| ctx.force_resync())?;
+                Err(AnkiError::SyncError {
+                    info: "".into(),
+                    kind: SyncErrorKind::ResyncRequired,
+                })
             }
         } else {
             Err(AnkiError::server_message(resp.err))
@@ -556,16 +563,17 @@ fn zip_files(media_folder: &Path, files: &[MediaEntry]) -> Result<Vec<u8>> {
 
         let normalized = normalize_filename(&file.fname);
         if let Cow::Owned(_) = normalized {
-            // fixme: non-string err, or should ignore instead
-            return Err(AnkiError::sync_misc("invalid file found"));
+            return Err(media_check_required());
         }
 
         let file_data = data_for_file(media_folder, &file.fname)?;
 
         if let Some(data) = &file_data {
             if data.is_empty() {
-                // fixme: should ignore these, not error
-                return Err(AnkiError::sync_misc("0 byte file found"));
+                return Err(media_check_required());
+            }
+            if data.len() > SYNC_SINGLE_FILE_MAX_BYTES {
+                return Err(media_check_required());
             }
             accumulated_size += data.len();
             zip.start_file(format!("{}", idx), options)?;
@@ -605,6 +613,13 @@ fn version_string() -> String {
     format!("anki,{},{}", version(), std::env::consts::OS)
 }
 
+fn media_check_required() -> AnkiError {
+    AnkiError::SyncError {
+        info: "".into(),
+        kind: SyncErrorKind::MediaCheckRequired,
+    }
+}
+
 #[derive(Serialize)]
 struct FinalizeRequest {
     local: u32,