Data not getting uploaded to the internal table from a text file throug BDC

There are two concerns.

One is in GUI upload.

The import file name should be of type string. But I am not able to declare a variable of type String. it gives me syntax error "Nested data type not allowed". Hence GUI upload throws run time error due to type mismatch.

Secondly, the data from text file is not read into the internal table. I am using tab seperator in my text file. I tried with different seperators and giving 'X' and '#' as field seperator in FM. But internal table does not get populated. My BDC program runs fine if I manually alter the data in the internal table while debugging.

Provide solutions for these