t.binary :file,:limit => 10.megabytes

Heroku给了我这个Postgresql错误：

An error has occurred,this and all later migrations canceled:

PGError: ERROR:  type modifier is not allowed for type "bytea"
LINE 1: ..."file" bytea(10485760)...
                                                         ^
: CREATE TABLE "files" ("id" serial primary key,"file" bytea(10485760),"created_at" timestamp,"updated_at" timestamp)

如何更改我的迁移,以便它与MysqL和Postgre一起使用？