How to Fix Mastodon ‘Could Not Save Post’ Error With Media Attached

You write a post in Mastodon, attach an image or video, click Publish, and see Could not save post. The post never appears on your timeline or in your followers feeds. This error usually happens because the media file you attached violates a limit set by your Mastodon instance or because the file itself is corrupted or unsupported. This article explains the exact causes of this error and provides step-by-step fixes that work on any Mastodon instance, whether self-hosted or managed by a provider.

Why Mastodon Rejects Posts With Attached Media

Mastodon instances enforce strict limits on media uploads to protect server storage and bandwidth. When you attach a file that exceeds these limits or uses an unsupported format, the server refuses to process the post and returns a generic Could not save post message. The error is deliberately vague to avoid leaking server configuration details to potential attackers. The root cause is almost always one of three things: file size, file type, or file corruption.

File Size Limits Per Instance

Every Mastodon instance sets its own maximum attachment size. The default Mastodon configuration allows 4 MB for images and 40 MB for videos. Some instances raise these limits, while smaller or stricter instances lower them. When you upload a file larger than the instance limit, the server rejects the entire post — not just the attachment. This means even a text-only post with an oversized image will fail to save.

Allowed Media Formats

Mastodon uses the Active Storage library from Ruby on Rails to process uploads. Active Storage validates file types against a whitelist. The supported formats are JPEG, PNG, GIF, WebP, and MP4. Uploading a TIFF, BMP, SVG, or MOV file will cause the server to abort the save operation. Some instances also block animated PNG files or files with incorrect MIME types, even if the extension appears valid.

File Corruption or Incomplete Uploads

A file that appears normal on your local device may be truncated or corrupted during transfer. This happens most often with files downloaded from messaging apps, email attachments, or cloud storage services that compress or strip metadata. The Mastodon server attempts to read the file headers to generate a thumbnail. If the header is missing or damaged, the server throws an exception and cancels the post.

Steps to Fix the Could Not Save Post Error

Method 1: Reduce the Media File Size

1. Check your instance media limit
   Open Preferences > Administration > Server Settings if you are an admin. If you are not an admin, ask your instance owner or check the instance About page for the media upload limit. Common limits are 4 MB, 8 MB, or 16 MB for images.
2. Resize the image to under 4 MB
   Use a free tool like GIMP, Paint.NET, or an online resizer. Set the longest side to 1920 pixels for photos or 1280 pixels for screenshots. Save as JPEG with quality set to 80 percent. This typically produces a file under 2 MB.
3. Compress video files to under 40 MB
   Use HandBrake or FFmpeg. Set the video codec to H.264, the resolution to 1280×720, and the bitrate to 2 Mbps. This keeps most 30-second clips under 10 MB.
4. Upload the resized file
   Attach the compressed file to a new post and click Publish. If the post saves successfully, the original file exceeded the size limit.

Method 2: Convert the File to a Supported Format

1. Identify the current file format
   Right-click the file on Windows and select Properties. Look at the Type of file field. Common unsupported types are TIFF, BMP, SVG, and MOV.
2. Convert images to JPEG or PNG
   Open the image in Microsoft Photos. Click the three-dot menu and select Save as. Choose JPEG from the Save as type dropdown. For batch conversion, use the built-in PowerToys Image Resizer.
3. Convert videos to MP4
   Open the video in VLC Media Player. Go to Media > Convert / Save. Select the file, choose the MP4 profile, and click Start. The output file will be in a format Mastodon accepts.
4. Test the converted file
   Attach the new file to a draft post. If the attachment preview appears correctly, publish the post. The error should no longer occur.

Method 3: Repair or Replace a Corrupted File

1. Re-download the original file
   If the file came from email, a chat app, or cloud storage, delete the local copy and download it again. Ensure the download completes fully before attaching it to Mastodon.
2. Open the file in an editor and re-save
   Open the image in Microsoft Paint or the video in VLC. Save a new copy with a different filename. This rewrites the file headers and often fixes minor corruption.
3. Use a file repair tool
   For JPEG files, use the free JPEG-Repair utility. For MP4 files, use FFmpeg with the command ffmpeg -i input.mp4 -c copy output.mp4. This recovers playable streams from partially damaged files.
4. Upload the repaired file
   Attach the repaired file to a new post. If the post saves, the original file was corrupted.

If the Error Persists After Following the Main Fixes

Server-Side Storage Is Full

Your Mastodon instance may have exhausted its available disk space. When the server cannot write new media files to disk, it returns the Could not save post error for any post with an attachment. Check with your instance admin or, if you are the admin, run df -h on the server to see disk usage. Free up space by deleting old media from the storage directory or increasing the disk allocation.

Browser Cache or Extensions Interfere

A corrupted browser cache or a misbehaving extension can corrupt the multipart upload request that Mastodon uses to send media files. Try posting from a different browser or from the Mastodon mobile app. If the post succeeds there, clear your browser cache and disable extensions one by one to find the culprit.

Instance Software Version Is Outdated

Older versions of Mastodon (pre-4.0) had a bug where posts with multiple attachments would fail if any single attachment exceeded the per-file limit, even if the total size was under the limit. Update the instance to the latest stable release. If you use a managed instance, ask the provider to apply the update.

Item	Image Upload	Video Upload
Default maximum file size	4 MB	40 MB
Supported formats	JPEG, PNG, GIF, WebP	MP4 (H.264)
Common unsupported formats	TIFF, BMP, SVG	MOV, AVI, MKV
Best tool for resizing	GIMP or Paint.NET	HandBrake
Server-side cause	Disk full or outdated software	Disk full or outdated software

The Could not save post error with media attached is almost always caused by a file that is too large, in the wrong format, or corrupted. By resizing the file, converting it to a supported format, or repairing the data, you can resolve the issue in a few minutes. If the problem continues, check your instance disk space and browser setup. Use the Mastodon mobile app as a quick test to isolate browser-related problems.