cubicweb #2526345 mimetype is text/plain not text/txt [done]

Error seen in web/views/management.py

prioritynormal
typebug
done in3.15.6
load0.100
load left0.000
closed by#7bacc4f21edc [web/views] bugfix: the mime type is text/plain, not text/txt (closes #2526345)