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Asked: 2026-06-02T21:57:05+00:00

I’ve got a server script receiving an uploaded file from Javascript. Client-side, using a

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I’ve got a server script receiving an uploaded file from Javascript.

Client-side, using a File object (from the W3C File API) and code similar to this line:

if (file.type.indexOf("text") == 0) { ... }

one can perform a check of the file type. Apparently, this uses a MIME type (which returns these strings).

In my journeys here through SO, I ventured across this worthy contributor, who maintains that MIME types are useless.

Are MIME types indeed basically useless in a file upload situation, and any type checking should therefore occur server-side?

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  1. Editorial Team

    The contributer is correct. You can’t rely merely on MIME type checking to truly validate a file. It’s only useful for quick lookups. For instance, on the client side, you can check the MIME type of a file before it is sent to the server, just in case the user chose the wrong file type, saving time and bandwidth. Apologies for the liberal use of commas!

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