The file someone sent you won't open: a quick diagnosis guide

23 August 2026 · 7 min read

A file that will not open on a Mac is one of five things: incomplete, mislabeled, in a format your Mac has no app for, in a container your Mac handles but with a codec it does not, or damaged. Check them in that order, because each check takes under a minute and the fix is different for each. Most “broken” files turn out to be the first three, and none of those involve anything actually being wrong with the file.

Check one: did the whole file arrive?

Downloads stop, AirDrop gets interrupted, and chat apps hand over a thumbnail when you thought you were getting the file. The file has the right name and icon and nothing inside.

Select the file in Finder and press Command-I.
Read the Size line. “Zero bytes” means nothing arrived; re-request it.
If there is a size, ask the sender what theirs shows. A video that is 48 MB on their phone and 31 MB on your Mac was cut off.
Look in the Downloads folder for a file with the same name and a .download or .part extension; that is the in-progress version, and the one without it may be a stub.

If the sizes differ, the fastest fix is a fresh transfer. If a fresh transfer is impossible, a truncated video or PDF can sometimes be partly recovered (see check five), but a truncated Word document or archive usually cannot.

Check two: is the extension telling the truth?

Files get renamed in transit more often than people expect. A web page saved instead of the file it linked to, a HEIC sent by a tool that relabeled it .jpg, an MKV renamed .mp4 to get past an upload filter, or a ZIP that is really a folder with a misleading name.

In Get Info, read the Kind line. Finder identifies files by their content in many cases, and “HTML document” on something called report.pdf tells you the download grabbed an error page.
If Kind simply echoes the extension, open Terminal, type file (with a trailing space), drag the file into the window, and press Return. The file command reads the first bytes and reports what the data actually is, extension or not.
If the reported type differs from the extension, rename the file to match, then try opening it again.

Renaming is the fix for a mislabeled file. It is not a fix for a file whose contents are in a format you cannot open; that is the next check.

Check three: does your Mac have an app for it?

macOS opens a broad set of formats with Preview, QuickTime Player, Pages and TextEdit, but the list has gaps, and a recipient with none of the sender’s software sees “There is no application set to open the document”.

Right-click the file and look at the Open With submenu. If a suitable app appears, pick it.
If only “App Store” or nothing appears, check the extension against the common gaps: WEBM, MKV, AVI, WMV and FLV video; FLAC, OGG, Opus and WMA audio; WEBP, AVIF, PSD and camera raw images; old .doc, .rtf with embedded objects, and .docx files from much newer versions of their application.

For most of these the practical answer is conversion to a format your Mac opens natively. Manatee accepts 122 input types across video, audio, images, raw photos, documents and subtitles, and converts to fifteen common targets: JPEG, PNG, HEIC, TIFF, MP4, MOV, GIF, MP3, M4A, WAV, FLAC, DOCX, RTF, TXT and SRT. Drop the file on its menu bar icon, pick a target, and the converted copy appears next to the original, which is never modified. Two honest limits: Manatee does not convert Pages, Numbers or Keynote files (those open natively on a Mac anyway, and export from their own apps), and it does not open password-protected files or remove passwords from them. For a walkthrough of the video cases, see MKV file won’t open on Mac: how to fix or convert it.

Check four: right container, wrong codec

This is the confusing one. The file is an MP4 or MOV, QuickTime opens MP4s every day, and this one says it is not compatible, or plays video with no sound, or sound with a black picture.

An MP4 is a container, and what is inside can be compressed with codecs macOS does not decode: HEVC on an old macOS, AC-3 or DTS audio from a camcorder or a download, MPEG-4 Part 2 in an old AVI, VP9 inside something renamed to .mp4.

In Get Info, expand More Info and read the Codecs line.
“H.264, AAC” means the codecs are fine and the problem is elsewhere; go to check five.
Anything else on the list above means a re-encode is needed. Converting to MP4 in Manatee produces H.264 video and AAC audio, which every Mac plays.

Container vs codec: why an MP4 can still refuse to play has the longer explanation and a list of codecs you are likely to meet.

Check five: is it actually damaged?

Only now, with the file complete, correctly labeled, in a supported format and supported codecs, is damage the leading explanation. Typical signs:

A video with the wrong duration, a freeze at the same timestamp every time, or gray blocks spreading across the picture.
A PDF that Preview reports as damaged, opens with blank pages, or stops partway through.
A Word document that its app says is corrupt or unreadable, often after a crash or a cloud sync conflict.
A ZIP that reports an unexpected end of archive.
A JPEG that shows the top half and then gray.

Manatee’s Repair mode handles corrupt video, PDF and Word documents, and performs container repair on ZIP and Office archives. It reads what is recoverable, writes a new copy alongside the original, and reports one of three outcomes: Fixed, Partial (for example, 34 of 41 pages, or the first eleven minutes of a video), or Not fixable. The original is never modified, so trying costs nothing. What it cannot do is invent data that was never written: a recording cut off by a dead battery ends where the recording ended, and a file whose bytes were overwritten on disk is beyond any repair tool. Why files get corrupted and what can actually be recovered sets realistic expectations.

Two cases that are none of the above

Permissions: a file copied from another user’s account or an external drive can show “You don’t have permission to open this”. In Get Info, expand Sharing & Permissions and give your user Read & Write, or duplicate the file to your own Desktop.

Passwords: a PDF, Office document or archive that asks for a password is not broken. The sender set protection and needs to give you the password. No repair or conversion tool should claim to remove it, and Manatee does not.

Questions

The sender says it opens fine for them. Who is right? Both of you, usually. Their device has the app, the codec or the complete file; yours is missing one of them. Run the five checks and you will find which.

Should I try repair before conversion? No. Repair is for damaged files, and a perfectly good WEBM is not damaged. Convert when the format or codec is the problem; repair when the codecs line looks normal and the file still misbehaves.

The file opens but looks wrong (odd fonts, missing images, shifted layout). That is a compatibility difference, not a broken file. Ask for a PDF version if the layout matters, or convert the document to DOCX or RTF and accept that complex formatting may shift.

Can Manatee recover a file I deleted? No. It repairs files that exist and are damaged. An undeleted file is a disk recovery problem, and the sooner you stop writing to that disk, the better the odds with a recovery tool.