Word for a person who uploaded a file?

The word 'uploader' is fine; many verbs in English are suffixed with -(e)r to describe a person doing the verb, e.g. baker, builder, dealer, driver, singer. 'Downloader' and 'uploader' are widely understood, and while it's not an official dictionary, Wiktionary even has it as an entry:

Noun

uploader (plural uploaders)

1. Someone who uploads files

I believe “uploader” is the most obvious, and therefore the right choice. You might prefer a different word if:

1. There is potential confusion about “upload” versus “download” (the terms are ambiguous in the context of peer-to-peer file transfers); or,
2. Your application needs to make precise distinctions between, say, the user account that physically sent the file and the person or company who was legally responsible for sending it.

In that case “submitter” would also work.

You could use "uploader" although I agree it sounds a little strange. You might also consider "source", "poster," or "contributor."

Alternatively, you could use the word "by" and say "uploaded by:" or "posted by:" or "contributed by:"

I would go with "submitter".
Also, can someone submit for a company, or for someone else? In that case maybe add a new field.