A Teams file upload can fail because of the file's name, not its size. Teams keeps shared files in SharePoint and OneDrive, and those two services publish a short list of characters that aren't allowed in file and folder names. If your file's name contains one of them, renaming is the fix — re-exporting a smaller version will not help.
That mix-up is most expensive with video, because video is exactly where people assume size is the culprit and start re-encoding before checking anything cheaper.
Why the file name is Teams' problem at all
Teams doesn't hold the attachment itself. Microsoft's limits and specifications page for Teams states it plainly:
"Because Teams runs on a SharePoint backend for file sharing, SharePoint limitations apply to the Files section within a Team."
The same page is equally blunt about what happens when that backend isn't in place:
"If you don't have SharePoint enabled in your tenant, Microsoft Teams users can't always share files in teams. Users in private chat also can't share files because OneDrive for Business (which is tied to the SharePoint license) is required for that functionality."
So when a Teams upload fails, the rules that decide the outcome are OneDrive's and SharePoint's. Microsoft's restrictions and limitations page for those services explains why names get checked at all:
"Certain characters have special meanings when used in file names in OneDrive, SharePoint, Windows and macOS, such as
"*"for wildcards, and"\"in file name paths. If a file or folder you're trying to upload to OneDrive contains any of the characters listed below, it may prevent files and folders from syncing. Rename the file or folder to remove these characters before you upload it."
Read the last sentence again — the official instruction is to rename before you upload. This is a pre-flight check, not something you negotiate with after the failure.
The fastest fix: rename the file, then try again
Microsoft publishes the blocked set under this heading:
"Characters that aren't allowed in file and folder names in OneDrive for home or OneDrive for work or school"
" * : < > ? / \ |Nine characters. The one that catches video files most often is the colon, because a colon reads as perfectly normal punctuation inside a timestamp or a title, and it survives being copied between machines.
The page adds a second rule that is easy to miss because you cannot see it:
"Leading and trailing spaces in file or folder names also aren't allowed."
A trailing space is invisible in every file listing you will look at. If a name looks fine and still fails, click into it and press End to see where the cursor lands.
There is also a list of names that are rejected outright, regardless of which characters you used:
.lock, CON, PRN, AUX, NUL, COM0 - COM9, LPT0 - LPT9, _vti_, desktop.ini, any filename starting with ~$Three more conditions from the same page carry their own qualifiers, and the qualifiers matter:
#and%— "Some organizations don't yet support#and%in names." This one depends on your tenant, so treat both as risky rather than banned.- Semicolons in folder names — "For Office desktop win32 apps: If you're saving an Office file via the Backstage view to a OneDrive or SharePoint folder, you won't be able to save the file if the folder name contains
;(semicolon)." That is scoped to saving from an Office desktop app, not to every upload. - Ampersands on Office 2010 — "If you're using Office 2010, you can't use
"&"in file and folder names."
Practical rename rule: strip the punctuation down to letters, digits, hyphens and underscores, keep the extension, and check both ends of the name for stray spaces. Team sync: Q3 review.mp4 becomes team-sync-q3-review.mp4 and uploads as a completely ordinary file.
Other causes, in order of likelihood
The size number depends on which route you used
If the name is clean, size is the next thing to look at — but Teams does not have one upload number. Microsoft's limits page carries different figures in different tables:
| Route | Figure Microsoft publishes | Where it appears |
|---|---|---|
| Chat | 100 MB, and a maximum of 10 file attachments | "Messaging → Chat" table |
| SharePoint / OneDrive storage | "File upload limit (per file) 250 GB", identical across all six plan columns | "Storage" table |
| Emailing a channel | "Size of each file attachment — Less than 10 MB" | "Emailing a channel" table |
Those first two numbers sit on the same official page, and Microsoft doesn't spell out the exact boundary between them, so check the official documentation for the figure that applies to what you're doing. On the attachment count, the page's footnote is specific: "If the number of attachments exceeds this limit, you see an error message."
If your video is genuinely over the number for your route, shrinking it locally is the quickest route out — and it avoids handing an unreleased recording to a third-party upload service on the way.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
This runs entirely in your browser tab: the file is read from disk, re-encoded on your machine, and handed straight back. Nothing is uploaded anywhere. If you want more control over the output, the video compressor exposes the same encoder with the knobs visible. For the specific case of a meeting recording that is too heavy to pass around, see Teams recording too large to email.
You're posting into a large group chat
Microsoft documents a separate failure that looks like a broken upload but isn't:
"When you try to share files in a Microsoft Teams group chat or meeting chat that contains more than 100 participants, the message might not be sent successfully when you select Send."
The stated cause is the sharing link type, not the file:
"This issue occurs if the default setting for file sharing is People currently in this chat. In this scenario, the type of link that's used for the shared files is Specific people. However, we don't recommend using this link when you want to share files with more than 100 users. When there are more than 100 participants in a chat, some participants might need to request permission to access the shared files."
If you're sharing to a chat that size, upload the file to SharePoint or OneDrive first and post a link with a broader sharing scope.
The name is fine but the path is long
Microsoft declines to give a number here:
"Different apps and Office versions have different limits, and the combination of limitations may be unique to your setup."
So there is no threshold to quote. If a deeply nested folder is involved, shorten the folder names or move the file closer to the root and retry.
How to tell it worked
Rename, then upload the same file again without changing anything else. If it goes through, the name was the problem and you have your answer in one attempt. If it still fails, you've cleanly eliminated the cheapest cause and can move to size and sharing scope with a clear head. Other Teams fixes are collected in the fix hub.
FAQ
Which characters are not allowed in Teams file names?
Microsoft's OneDrive and SharePoint restrictions page lists these as not allowed in file and folder names: " * : < > ? / \ |. The same page states that leading and trailing spaces aren't allowed either.
Why does my Teams upload fail instantly with no progress bar? A blocked character or a leading or trailing space is checked against the name, so nothing has to be transferred before it can fail. Microsoft's guidance is to rename the file or folder to remove those characters before you upload it.
Are there file names that are rejected even with legal characters?
Yes. Microsoft lists .lock, CON, PRN, AUX, NUL, COM0 through COM9, LPT0 through LPT9, _vti_, desktop.ini, and any filename starting with ~$ as not allowed for files or folders.
What is the actual file size limit for Teams? It depends which table you read. Microsoft's Teams limits page lists 100 MB in its chat messaging table and a 250 GB per-file upload limit in its storage table, and separately caps attachments emailed to a channel at less than 10 MB each. Check the official documentation for the one that applies to your route.
My recording failed to upload to OneDrive — is that the same problem? No, that's a different path with its own handling. See Teams recording failed to upload to OneDrive.
Rename first, compress second — in that order, most Teams upload failures resolve in a single retry. Everything on this site that touches a video file runs locally in your browser; the full set is at Clapr tools.
Character and name rules cited from Microsoft's OneDrive and SharePoint restrictions page; Teams figures from the Microsoft Teams limits and specifications page, checked 16 September 2026. Microsoft revises these pages — verify against the official documentation before relying on them.
