POST API

You will have to specify a folder in which you would like to keep your document in. 

You can use this template

suppose an user sends a document using POST API . how can I map his details with the document he sent in Database

You define the folder in the properties dialog.

Check this example:

docs.appian.com/.../passing-a-web-api-document-into-a-process-model.html

Then what datatype can be define to store the value of image

Did you read the documentation? Process variables and CDT fields can have the "Document" data type. As Appian stores files always in the file system, the field will internally hold the integer ID.

Did you read the documentation? Process variables and CDT fields can have the "Document" data type. As Appian stores files always in the file system, the field will internally hold the integer ID.