仿照Bootstrap的input 修改 upload上传图片的样式

html：

<div class="form-group">
    <label class="col-sm-3 col-sm-6 control-label">图片</label>
    <span class="problem-must"> </span>
    <div class="col-sm-8">
        <a class="fb-upload" href="javascript:void(0);"> 
            <input class="form-control" name="pic" type="file" accept=".gif,.jpg,.png,.jpeg" /> 
            <span class="fb-img">上传图片</span>
	</a>
    </div>
</div>

css:

.form-group input[type="file"] {
    cursor: pointer;
    opacity: 0;
    position: absolute;
    top: 0;
}
.fb-img {
    border: 1px solid #ccc;
    border-radius: 4px;
    box-shadow: 0 1px 1px rgba(0, 0, 0, 0.075) inset;
    color: gray;
    display: inline-block;
    height: 34px;
    line-height: 1.42857;
    padding: 6px 12px;
    text-align: left;
    transition: border-color 0.15s ease-in-out 0s, box-shadow 0.15s ease-in-out 0s;
    width: 100%;
}