For now, upload the file, compute its sha1, dismiss the content, check if sha1 in swh storage exists.
Answers the query.

Sample:

$ curl -X POST -F filename=@/path/to/file http://localhost:6543/api/1/uploadnsearch/
{
    "found": false,
    "sha1": "e95097ad2d607b4c89c1ce7ca1fef2a1e4450558"
}%

(code 200)

Regarding the limit, olasd and i spoke of apache's settings for this.

In the mean time, it's flask which limits it.
This is a parameter in swh-web-ui's of 16Mb for now.
(Throws a 413 entity too large if too much -> tested with a tarball of 21Mb)