[SalesForce] JSON: Cannot deserialize instance of date from VALUE_STRING

You have to cast it into a List<Object>, and for each element in the list, you can access the values as a Map<Object,Object>.

I just tested serializing and deserializing an attachment in Salesforce, and JSON.serialize gave me something like this:

[{"attributes":
 {"type":"Attachment",
  "url":"/services/data/v37.0/sobjects/Attachment/00PXXXXXXXXXXXXXXX"},
  "Body":{
    "asByteArray":"AA==",
    "inputStream":{},
    "length":1,
    "maxToKeep":524288},
   "Id":"00PXXXXXXXXXXXXXXX"}
]

However, it wouldn't convert it back into an actual attachment, giving a similar error about not being able to handle base64 data.

This suggests to me that you're going to have to manually deserialize the values, perhaps like this:

@HttpPost
global static String createMerchandise(String name,String id,List<Wrapper> attachs) {
// Skipping prior code
Attachment[] attachments = new Attachment[0];
for(Wrapper item: attachs) {
    attachments.add(
        new attachment(
            name=item.name, 
            body=EncodingUtil.base64Decode(item.body), 
            ParentId=m.Id, 
            ContentType=item.mimeType
        ));
}
insert attachments;

You'll have to adjust your JSON to match. Wrapper is just a plain class:

class Wrapper {
    String name, body, mimeType;
}