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I've been exporting my vector layers to GPX format succesfully until 2.18.10, now I get the following error when trying to do so:

Export to vector file failed.
Error: creation of field fid failed (OGR error: Field of name 'fid' is not supported in GPX schema. Use GPX_USE_EXTENSIONS creation option to allow use of the <extensions> element.)

I've got a few non-GPX columns in my attributes tables (fid, fstatus, etc) that are not natively supported by the GPX format, so I make sure to set the GPX_USE_EXTENSIONS field in the Save As gui to YES. This was working fine in versions 2.18.9 and below with the non-GPX fields get saved to an <extensions> section with an <ogr:...> prefix. But with 2.18.10 this operation fails.

Steps to reproduce:

- Add a new vector layer from a GPX flie containing extension data (see attached)
- Right click on the newly added layer and select "Save As"
- Select GPX as the output format and assign a file name to the output
- Assure the GPX_USE_EXTENSIONS field is set to YES
- Click OK to save the file

Expected behavior : the layer data is save to a GPX file
Observed behavior : An error dialog appears and the created GPX file contains no geometry