Listen if any file size is unknown then Opera Mini can't download any unknown file. If download sites is password protected example (401 Unauthorized) then Opera Mini can't download any unknown file size its session problem because Opera Mini hit direct download link and link is encrypt or password protected then opera mini can't download any unknown file size.
example.com/download.php/njp9kmV8/NQPbYMuy/3xm6G3yM/uKVE3vyw/2FDxUSD/J3F7Nn1p/QLkvxobZ/r4VI2Fx/
this link is encrypted then opera mini hit direct link then opera mini can't download any unknown file size.
here is example. 1. go to (www.hideme.be) 2. then go to any download web site then download any file size via (www.hideme.be) with OPERA MINI then you see what is the main problem.
GRAMEENPHONE BANGLADESH opera mini server problem (This problem started 10/08/2015)
Please fix Unknown file size download problem @ OPERA TEAM