This can occur when the .csv extension is associated with a different type of file in the Applications list. That's the list on the Settings page where you specify what application should open various types of files. Can you see CSV listed with any other content type there? This article has the steps to get to the list:

Manage file types and download actions in Firefox

If that is the issue, there isn't a convenient way to edit the list. (There are some inconvenient ways...)

I would think that Firefox sends the file to the server with a MIME type that the website doesn't recognize as a valid MIME (content) type. I'm not sure if you can catch this in the Network Monitor.

• https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/

You can check handlers.json in the Firefox profile folder to see if there is a reference to CSV.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I would think that Firefox sends the file to the server with a MIME type that the website doesn't recognize as a valid MIME (content) type. I'm not sure if you can catch this in the Network Monitor.

• https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/

You can check handlers.json in the Firefox profile folder to see if there is a reference to CSV.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I'm sorry, I'm just an end user. I don't understand your reply. But thank you.

This can occur when the .csv extension is associated with a different type of file in the Applications list. That's the list on the Settings page where you specify what application should open various types of files. Can you see CSV listed with any other content type there? This article has the steps to get to the list: Manage file types and download actions in Firefox If that is the issue, there isn't a convenient way to edit the list. (There are some inconvenient ways...)

Thanks, but the problem I'm having is occurring during an "Upload" process. Your solution address how Mozilla Firefox handles "Downloaded" files.

jscher2000 - Support Volunteer said

This can occur when the .csv extension is associated with a different type of file in the Applications list. That's the list on the Settings page where you specify what application should open various types of files. Can you see CSV listed with any other content type there? This article has the steps to get to the list: Manage file types and download actions in Firefox If that is the issue, there isn't a convenient way to edit the list. (There are some inconvenient ways...)

Thanks, but the problem I'm having is occurring during an "Upload" process. Your solution address how Mozilla Firefox handles "Downloaded" files.

The information about what file extensions are associated with which content types is a part of the puzzle. That is why I suggested checking the list on the Settings page, and cor-el mentioned the file which stores that information.

You can inspect the MIME database key with the registry editor (regedit.exe) and do a search for that MIME type (file extension) via Ctrl+F. Be cautious with editing the registry as there is NO UNDO possible: all changes are applied immediately. You can export key(s) in the registry editor before making changes. You can check specific file extension keys (e.g. .csv) in the registry with the registry editor.

• HKEY_CLASSES_ROOT\.xxx

You can check possibly linked MIME types in the MIME Database registry key.

• HKEY_CLASSES_ROOT\MIME\Database\Content Type\