F5 not Identifying Parameter in Text/Plain Upload

text format doesnt mean it's safe.
for example, sql injection attack, credit card data theft, etc. are also text.

for your case, you can create local traffic policy to disable asm or set specific asm policy for the upload file traffic.

and btw, content disposition header is usually for http response, not request.
that might be the cause why asm block that http upload request
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition