After an agonizing evening of research and tests solution was finally found. Simplest way to edit a file form field style is inserting the following css code in you css style:

input[type=”file”] {
text-align: right;
opacity: 0.8;
font-size:9px;
float:left;
max-width:400px;
size:10;
}

Replace “file” part with any field name you want to edit its style easily.