cmd.exe supports a for loop, like:
for %c in ( file*.txt ) do process %c
It supports a fair number of options for things like getting only the base name of the file in question, so if (for example) you wanted to the .txt files and produce a file with the same base name and the extension changed to, say, .dat, that's pretty easy to do.
In this case, the apparent intent (gleaned from multiple comments) is to step through some files named tweetNNN.txt, where NNN is some number from 1 to 100. The content (not just the name) of that file is then to be passed on a cURL command line as data in a request.
The easiest way to do this is probably to use the @ character on the cURL command line, something like this:
for /l %c in (1, 1, 100) do echo "language=english&text=" > stage.txt
&& copy stage.txt + tweet%c.txt
&& curl -d@stage.txt text-processing.com/api/sentiment/ >> results.txt
(Note: I've formatted this with line-breaks, but it needs to be entered as a single line).
I put together a quick test, using a couple of files:
tweet1.txt: "what complete crap. hated every minute"
tweet2.txt: "Best movie of the years. Loved it"
Running the previous command in a directory containing those two files produced a results.txt containing:
{"probability": {"neg": 0.82811145456964252, "neutral": 0.18962854533013332, "pos": 0.17188854543035748}, "label": "neg"}
{"probability": {"neg": 0.10467714372495518, "neutral": 0.080508941181180751, "pos": 0.89532285627504482}, "label": "pos"}
That seems a close enough fit with the content of the files that I think we can safely conclude that the text was analyzed as desired.